It's really simple once you know:
1 buy a cheap foam camping Mat.
2. Most plates are 10 wide by 12 tall, angled at the top song would start by cutting a 10x12 square, then lay it over the PC. To work out where your want the angles. The foam can be cut with normal scissors or a Stanley knife with adult supervision.
3. Test fit and trim to carrier.
Personally I don't use any thing in the back, and double up the Mat to fill the void.
It won't weigh the same as real plates, but will help hold the shape of the rig.