That works. You can also leave tabs on the perimeter cut and finish the cuts with a hand saw or other small powered saw. I haven’t looked at the big box files, but did see one a couple months ago. Really nice looking printer! Since you can’t cut a square inside cut, you’ll need to look into putting in some corners. As for the bit selection, either a down cut or up cut will work. Depends on which side of the ply you want to look nice. Up cuts may lift the plywood in the middle, so watch out for that. Down cut bits should never be used to make holes, you can start a fire, think about how fire bows work on those survival shows, same thing will happen. They do make up and down cut bits, but they’re expensive, and I’ve not tried one yet.