So, here is one of the best tips I can give to a caregiver of a casted baby... DOUBLE DIAPER!
Before we started double diapering we had a leakage incident... oh man. That poop leaked right into his cast and the cast just soaked it up! So gross, I know, but I have to share in order to make my point. I went to get Merritt's cast changed the very next day - the earliest appointment I could arrange. He already had a little rash on his leg! It happens so quickly.
From then on, we double diapered. Basically, put on the first diaper and then a second. MAKE SURE the edges of the second diaper are on the outside of the first - offering an extra layer of protection. At some point, I started using a size larger on the outer diaper. You don't have to throw away the outer diaper every time you change the baby. Just if it gets leaked onto... which will happen!