How flat and stable is melamine?

Melamine is pretty darn stable. I’d check it by eye for
flatness – if it looks flat enough it probably is flat enough
for a cut off box. It can warp if stored badly.

I’d edgeband it.

DON’T get it wet and you’ll be fine…my old Delta Unifence extension is made of it and I made my outfeed tables from an old computer desk made of it. And IKEA has made $$$millions from it (or I guess Swedish Kroners). Your rear fence will help keep it stable.

I’m not familiar with the Kreg system. Guess I’ll have to look it up. Absent that thing, 2” high is not enough IMHO. I’m going to make a new one with a 5” rear for stability and safety.

Be sure to seal any exposed/raw edges. I would use shellac or varnish. Keeps any moisture ingress to a minimum.
Bill

-- bill@magraphics.us