What’s good for a bottomless tank

I’m in the uk

So I have axlotals and I don’t want the bottom of the tank to be bare

Bare on the bottom is good for the Axlotal cuz it’s more cleaner But I want something other than sand or gravel because of the impactions they may get

I don’t want sand / gravel Maybe a mat of some sort ? I wouldn’t mind getting non sharp slates but it’s where to get them from

Could you give me a shop or anything please I’ve been looking everywhere

My tank is I think 120 length by 41 width