I agree with Orraloon. I think the ply frame as sketched would be too flimsy. If you go that way, I suggest doubling up on the thicknesses (to 36 mm, two sheets glued together or one thick sheet) of the sides and the back, and also make the back full height without that big cutout. If you need the cutout for a motor than frame it with something thick.
It's really better to frame it using 100 x 100 legs and at least 35 mm top. You don't need to do fancy joinery, since if you fill the sides in with 18 mm ply that will make the whole thing stiff enough.
Weight low down and stiffness are important. If you can attach it to a wall that is also very good.