Being completely honest with you and not even exaggerating one little bit, I'd dump the crushed walnut shells out right this minute, no matter what I'd get it out of there immediately, and then you can just put down clean paper towels and replace them as he soils them. Worry about buying non-adhesive shelf liner, tiles, carpeting, or another permanent, solid substrate whenever you have time to do so, but do not hesitate to get that junk out of there right now, I don't care if it's midnight. I'm serious. I have seen that stuff literally kill so many healthy beardies I can't even begin to tell you how many. Adults and babies both, but usually it's beardies between just hatched and a year old that die suddenly and without any warning at all due to either a bowel obstruction or more often than not internal bleeding because the sharp corners and edges on that garbage literally cuts their gastrointestinal tract apart. We're talking hundreds of healthy beardies dying suddenly and quickly without warning.
I know I sound dramatic, but if you search "Bearded dragons crushed walnut shells" on Google, or just search "crushed walnut shell" on here, you'll see why I'm being so dramatic pretty directly.