I am not familiar with the Flexion Extruder but if it works, I say go for it. As for the glass, I don't put anything on it. Ninjaflex sticks really, really well to the bare glass. I also set the bed temp to to 40.

As a hobby, I restore old outboard motors and ninjaflex makes great seals, bumpers, rubber gaskets, and washers. I use it a lot!!!