I have cut down front wings earlier too (Zeeko wings), to make smaller and faster wings.
But all, inclusive my flat rear stabilizer, has been full carbon.
I had an early Ketos/Takoon prototype stabilizer, totally flat because it used a stab for yaw stability and nice tracking in turns.
So as the newer stabilizers have swept tips (which makes out for the stab, just less drag overall), it was a no brainer that the old flat wing should be cut down to an XS or XXS wave stabilizer - and it worked.
PS: Tried to sell the original non cut stabilizer at first (got new ones), for almost nothing, a give away symbolic price if anyone needed one - but most are afraid to use something new, and the homebuilders want to do it all themeselves and not get something pre-manufactured I found out, and it makes sense
If there was/is a core in some wings, I would cut it down first, then sand it all down to the perfect profile, and glass it over and sand it clean, and paint it black.
It would be a lot more fragile though, risking getting it ripped open if you scratch on the bottom
I dont think (m)any has foam core close to the tips - it would not be good at least, as you can not just sand it down when cuts.
Prefer to have both stabilizer options, as advantages/disadvantages of both the swept tips and the flat ones.
Might change in time which I prefer the most - we will see.
I am still just an old geek with limited talents, having fun out there