I fly almost all low-power stuff too, and when I want to make custom CRs I reach for a chunk of picture-framing matte cardboard.
You can get it for free at your local craft store (they always have a pile of it in their trash from framing someone's pictures). I explain to them that I use it to build models, I only need small scraps, do they have any (of any color) that they are throwing out? Usually they will come back with pretty good sized pieces that will last me for dozens of CR jobs.
The stuff usually has one face that is really smooth, great for marking and drawing your patterns. You can quickly rough-cut a small chunk using shears. It trims easily to final shape with an Xacto blade. If it is too thick you can peel off some layers of paper. You can soak a little CA into the edges to reinforce them, and the finished CR is plenty strong. Try it!
And welcome to TRF!
