That size will be fine. I have geckos in two tubs currently, and yes, plain window screen mesh will work fine. (I'm not sure if there's a specific name for it, I think it's just called screen mesh). I've seen it at many stores - Home Depot, Lowe's, Rona, etc. Just avoid metallic screen as it will rust in the humidity.
Then just cut out a hole and hotglue (or whatever method you prefer) the mesh to the outside of the hole.
I buy all my fake plants from either Michaels, Walmart, Dollar Stores, etc. Just wash them well and you're good to go. I also like to buy suction cups - I remove the metal hook from each one, use a plastic twist-tie to fasten the plant to the cup, and then I can use them along the sides and tops of the tubs so they hang down.