discovers adventurers intended for a 'bad air' world.
In the Halloween and autumnal seasons, you can often find orange table covers for sale.
I found this one at Dollar Tree for $1.25 USD in the seasonal section. It is 4.5' x 9' and is made of thin plastic.
Spread out on a table, it's not terribly impressive. The wrinkles aren't too bad, but I wonder if a hair dryer might help smooth them out?
Now throw down some hills, and you've got a alien lava planet! Above, you can see my Neo-Sovs scouting prior to battle.
Or in a fantasy setting, place some terrain on top, and you have a lava pool or river or lake. Above, Midge is quite worried about the depths of this dungeon!
You could mark it with markers, or it might even take paint (but I'm not sure how long the paint would adhere, given the flexibility of the cover). Of course, you can cut it to size.
When you're done, fold up the table cover and store in a large ziploc. Or discard it, it's cheap!