I have a few, but never use them. The smallest it 6" x something, the largest 18" x something.. Always found them too small, that they slip & slide over your work surface, and spilt glue ruins them. I spilled acetone or some other solvent on one of them, so the lines all disappeared, and it's all wonky & wavy now..
From my R/C plane days, I use a sheet of gyprock on my build table. (and my table is a sturdy 4' x 5.5' top, topped with an old door: long & flat!) The 4' x 8' sheet of gyprock is cut into quarters, so I have a stash near by! (in thirds actually) And how much is a piece or sheet rock?! $12?!
Gyprock is sacrificial. And, you can stick pins in it (great for when you need to make a balsa skins 12" x 8" - like on a certain MDRM..) When it gets beaten up, flip it over, or replace it.. I've used gyprock for my stained glass projects too: pin a fence when & where needed, and it can take the heat from an 80W iron!
And recently, I've acquired an old piece of glass (tempered - bonus!) that I cut on. Cutting paper or the like (Monokote!!) on glass with an X-Acto is super smooth & doesn't dull the blade as you would think..
Glue globs scrape / chisel off either quite well..
But then again, I also use an hobby Olfa knife with snap-off blades..
https://olfa.com/collections/professional/products/9mm-sac-1-stainless-steel-graphics-knife
Doesn't roll, blade retracts, and I can snap off a blade whenever I need to.. Replacement blades come in a pack of 5, and has 7 snap off blades.. ( $$ saving!)
Of course, people get them, thinking they can take punishment & abuse. They then try to cut a 1/4" sheet of balsa with a box cutter, in one pass.. gouging the cutting mat beyond repair / intended use
Get an old desk or something sturdy. A TV tray is meant for light duty, and it meant to fold up. You will soon have it fold on you, and at the worst possible time!