Hi there.
DS1 was in a spica cast for a month after fracturing his femur. Height of summer too!
I put a NB sized nappy tucked into the cast, then a large one over the top of it.
I'm afraid it did smell, but I didn't use anything to deodorise it as DS1 has many sensitivities and I didn't want to aggravate his skin.
I found useful toys that could be played with lying down - he loved his mobiles, his big sisters leapster, mirrors etc. I also bought one of those sit in gym things, because he could only be upright or flat, couldn't sit. But he didn't have a bar.
The nurse finishing off the plaster for me gave me a couple of rolls of the shiny tape (slick?) to put around the edges, keep it well padded.
DS did have a lot of sores, but he had been in traction for a month before he went into the cast, and he already had some. I just kept him as clean as possible. There was lots of room between him and the cast. In fact, he used to hide things in it, the toad!
HTH a little, massive ((hugs)) for you. xx
Oh, also, a Micralite is a really good stroller to use because of the shape of the seat.