Back on the topic of this wire mesh cloth material, I will only buy from 1 of 2 places:
Mcmaster Like you said - just watch the prices on them because they obviously do not manufacture this stuff, therefore the prices can go up and down a lot.http://www.bwire.com/
- They manufactuer this stuff and have 2 large warehouses full of inventory. They can sell you small pieces or rolls at the best price for material here in the USA.
The 60 x 40 medium mesh size should be fine for something like this. Stainless steel is obviously the material that you are going to be looking for to avoid any rust and corrosion. May want to buy a piece a little bit larger than the 12" x 12" square that you are thinking about. More to play around with in case of mistakes.