for the one i made i put a couple of large holes in it then a couple of small ones with a dremal drill bit and then i covered them with a mesh screen and silconed that into place with tape over the edges to keep the screen from fraying.

i recommend to use zoomeds terrarium mesh its cheap and wont fray like the bulk ones from a hardware store. (edited: I found out about bulk store mesh screens fraying the hard way by almost had my reptile escaping)

if money isnt much of an issuse and you want it to look nice you could get a second piece of plexiglass have vent holes drilled into the same places for both have have the mesh screen between them silconed together.

link to zoomed mesh screen Terrarium Mesh