I adopted an adult male glider back when I was in middle school. He was too old to bond and I basically made his cage a retirement home. I couldn't cut his nails because even after a year of trying to bond with him he wasn't really interested. He pretty much just tolerated my presence lol. He did not tolerate me cutting his nails though so I feel your struggle. I covered most of the branches and other toys with an abrasive insert (basically sandpaper with a sticky side). Just be sure it has a glider-safe adhesive.
Also make sure the gliders won't slip off of wherever you put it ex: don't coat the outside of their pouch with it because they can't hold on to it like fabric and they will slip off). The same goes for the sides of their cage. I had a stupid friend who coated large sections of the cage walls with it hoping the glider would jump from the other side of the cage on to the sandpapered sections. Unfortunately, the glider tried from too high up in the cage and because he couldn't grab the sandpaper, he just fell to the bottom over and over. He noticed quickly and luckily the glider wasn't injured.
I also read that making logs (branches) of concrete and putting them across the cage works well. The rough texture of the concrete files their nails when the run across the log. I think this may be your best bet.
Hope this helps. Its been a while since I've owned a glider. I put a post up recently with some questions since I plan on getting some again soon. There might be something new on the market to help with your problem, but if not I hope this helps!