You can use pieces of cork bark, and glue them on with silicone, or some of that rubber shelf liner, the stuff that grips and has little holes all in it. I just glue it down with silicone, or lay any sort of rough textured tile on the glass.
Most of my above tank basking platforms are wood, so I don't use anything.
well just spent over an hour trying to coax myrtle and bubbles onto thier new shelf, picking them up and putting them on there only for them to jump back into the water tried tempting with prawns - no luck so gave up for tonight...............
THEN son plodded down the stairs and said 'that was cool your turtle sitting under that lamp' WHAT i shouted and yes this morning whilst i was at work he was basking away!!!! lol lol lol