You can easily cut or 3d print bigger palm rests out of you favorite material. (When you overlay a flat board over the steel plate, the batteries still fit in just fine.)
E.g., similar to: https://forum.ultimatehackingkeyboard.com/t/wooden-palmrests-for-uhk80
I am now experimenting with 5mm plywood, which looks much less premium, but is easier to process, and still is a great improvement over rubber…