Model Rocket Guide Ring for cutting tubes

This is just a simple ring to go around a model rocket body tube. Estes sells a kit like this, but that'd be expensive for a classroom, so if you need a dozen guides for cutting BT-50 tubes this might be better.

WARNING: I only tested the BT-50 fit (I couldn't find another body tube in the house)
WARNING 2: My printer may need a different "FUDGE" factor than yours. If it's too tight or loose, open the scad and change the fudge.

I was also thinking it'd be "easy" to make adapter rings t