Haven't made one with a bottom placed fan yet. On theorethical considerations if you are right handed place the opening on the rear left hand corner. That's where most of the overspray goes. Don't place it in the center because that's where your workpiece will be and will cover the outlet.
I like this bottom fan idea. Its a good way to mount the fan, filters, compressor, store paint supplies, etc. The fan equipment can be mounted permanently under the tabletop. I make my spray booth from cardboard cartons that I dump once it becomes too messy. With a bottom fan set up all I will need to do is to cut a hole in the cardboard to match the tabletop fan. My present top fan set up requires that I get reasonably strong cardboard boxes and even then I have to add wood strip reinforcement to mount the fan, filter and switch. With the bottom fan set up I can use any flimsy cardboard box and dump them as often as I like.