I've read several sites talk about how to diaper a child in a spica cast. I have yet to post anything about because I figured others have covered it even if I do it a bit different. However, I've decided it's time share a bit of what I've learned from personal experience. From what I have read you should us a size 1 or 2 diaper; I would like to strongly disagree. Dr Brock's nurse gave me a paper before leaving the hospital that says use a diaper one to two sizes smaller then the one they wore pre spica. I totally agree! When Kaitlyn poos it can go up the back, front, or even in the sides of the cast. A smaller diaper allows the poo to freely go up. A bigger diaper pulls the poo out of the cast as you remove it.
Another thing I've learned has been about diaper styles. In the hospital we used size 3 Pampers; they worked well. If we didn't put a diaper over the one inserted into her cast it would fall out because of the smaller size. On the way home from the hospital I bought size 3 Huggies; these were a bad purchase. Huggies have a lot of elastic in the back which pulls it all together preventing it from staying nicely tucked into the cast. If you have a smaller opening in your cast then more power to you on using Huggies. I don't suggest them. My suggestion is to save those Huggies until your little one is cast free. Our third package of diapers were size 4 Luvs. Luvs are our normal brand. For someone who hasn't ever used Luvs, they are cut like Pampers. The larger size is working great!!! I have been thankful on more then one occasion we're using the bigger diapers. By the way, before surgery Kaitlyn was in size 5.
For a while we used the shell to her cloth diapers to cover the diaper area and to hold the inserted diaper in place. The more she started getting around the harder the cast was on the cloth diapers. We moved to a simple rotation with disposable diapers that is working well. We insert a size 4 diaper (with a super absorbent pad) and strap a size 4 diaper over top her cast. Did you know those diapers can stretch? If you are using a really cheap diaper that doesn't have stretch... pay a little more. So your child is diapered. At the next diaper change you take off the outer diaper and set it aside. The inner diaper and pad are thrown away. The outer diaper now gets the tabs pulled off (you don't need those scratching your little one), a super absorbent pad placed inside, and the combo inserted into the cast. Now diaper outside her cast with a fresh diaper and you're done. Rotating the diapers keep you from throwing away your money on a larger diaper to cover the cast. The stretched out tabs are going to be pulled off anyway so don't worry about the stretch you put on that diaper.
Diapering at night... Even when they aren't in a cast nightly diapering can be a challenge. Since being in the cast Kaitlyn seems to pee more at night. I have yet to do it, but putting the mattress at an incline (head up) will cause the pee to run down into the diaper rather then up the cast. We had a lot of trial and error at night until we learned what really works for us. I put two pads in Kaitlyn's nighttime diapers and I use a size 5 diaper. The first time or two I tried this it did okay, but I still had a partially dry diaper that leaked. I thought about buying different pads at night that didn't have a plastic back, but then came up with this...
I cut two pads up the back.

I pull the back open a little so there's a nice gap for all the pee to flow through to the diaper.

Here's the diaper with two pads, cut up the back, as close to side-by-side as I can get them. The smaller the diaper you use the more these two will have to be overlapped. Because more pee runs to the back I insert this mega diaper with the pads starting at the back cast edge.
Taping the cast has little to do with the actual diapering process, but it's important. While I had the camera out I thought I would take some shots of her taped cast.

All the white you see is waterproof medical tape. It comes in different sizes, but mine is 1" wide. I tried the slimmer stuff and goodness was that a pain to put on. Under all the white is a Gortex lining that covers the underside of the cast, some foam, and gauze. If you don't tape the Gortex within a day or two of the child coming home from the hospital the lining will rip. You don't want this happening. On her second cast the Gortex ripped on both knees before I could get it taped down. Thankfully I was able to pull it together and tape it down. Once you have waterproof tape all around the cast openings you then need to add Duck tape to hold it on. The waterproof tape sticks well to the Gortex, but not the cast. Duck tape comes in most to all cast colors. As you can see the hospital Kaitlyn had her surgery at uses a darker purple then Duck tape supplies a color for. However, their pink is spot on. As you can see from the front shot the tape between Kaitlyn's legs is damaged. From time to time I have to take off the Duck tape (carefully) and add more waterproof tape, then more Duck tape. Near the end of her being in the last cast I wouldn't take off the old tape, I would just add to it. When she had her cast down to her toes I still put both tapes on the toe area. I also put Duck tape around her ankle/heel and knee areas to protect them from all the crawling she was doing. I found myself almost nightly adding more tape to the knee of her one fully casted leg so I started putting a man's tube sock on that leg. My main reason for taping the knee and heel was to keep more cast from being applied to her existing cast, making her even heavier. Heaven knows I don't want her heavier.
EDIT: Near the end of Kaitlyn's cast time I realized we still had a lot of size 4 diapers. I swapped the size 5 at night for the 4. We never had an issue with leaks with the smaller diapers. I think the pads were the trick not the size of diaper.