>>961901
Super overbuilt. Wasting lots of money. This is what happening when framers think they can be cabinetmakers. Putting the wood on top and screwing down is stronger but its going to be ugly as fuck. Miter joints are your friend. Next time ask us where to begin because you have purchased framing timber for cabinetry and wasted a bunch of money on the wrong materials for the job.

>>961901
You'd want to have the top member on top of the side members. Think about the shear force you'd put on the screws if you did it the other way around.
I'd put a rabbet on the side members and rest the top piece in the rabbet then glue and screw.

Build the box on the ground and make sure everything is square. Plywood on the back will keep it square once you have the box built.

Personally I think the lumber you have is way overkill but it's your project. Any plans for doors to keep dust out?

>>961915
>over built
He's talking about butt joints and screwing into end grain, nothing about this says over built to me. Also, while a miter may appear nicer I wouldn't use it as a joint that may be put under stress, which these ones will due to the way they'll be positioned.

Since you've already committed to this op, get a 2x2 and use it as a brace in each inside corner. It'll be ugly but it'll be the easiest and cheapest way to make it stable. Orientate it that you can screw through the face grain of the outside board into the face grain of the 2x2. If you just screw into the end grain of the outside board it'll work its way out.

A rabbet and glue would be nice, but it may be out of your skill set. If you feel comfortable you can pull it off I'd do that.