There are cheaper alternatives. A lot of ebay sellers provide custom prints, with free shipping. Material are often silk or pvc. The silk ones are usually very good, while the pvc might be a bit dull in colour. The silk prints need to be carefully glued to something.
I’ve tried cheaper ones in the past. Often they end up looking crap. I had some space mats printed on banner pvc once and they ended up in the bin. I’ll happily spend a bit extra for a neoprene mat. Not really viable for enough mats for an event though I guess.
This album has a rather slick looking sump sea/flooded dome ideas. Looks like it is some black plastic liner/sheet with nothing else done to it. Potential experiment in the pipeline to glue some down to a board and then paint it a bit more toxic looking maybe with homemade inks to keep it looking black still.