Why would you wanna clamp anything???
Just put the box together as usual.
Drywall screws would break the wood(trust me) if you are trying to add the sides to the panel with drywall screws.
Also are you very limited on hardware locally?
Odering hardware online seems pain in the ass.
i avoid anything ordering online(takes forever damn it).
Also what design is your panel?
if you did not put anything together,then i suggest you make somethin looks similer to original killer instinct control panel.
this design of panel should be fairly easy.and you are trying to make angles too right?
i might not explain right but here goes anyways,
first cut 2 exact same piece for the panel and the bottom(exact same size).
then for the front wood,you cut a smaller piece wood and for the back you cut a bigger piece.
that way you have an angle.after you put together everything,its time for the sides..all you gotta do is cut the angles sides pieces and use VERY THIN tacks.i built several different design and this type came out very good and was easier then any design ever.i might have not explain very well but you get the idea

.
you can also use some putty to cover up the tacks that might show and then paint.or laminate the sides just like mine.

you see one big piece of wood.that one goes in front and you see 2 bigger pieces,they go in the back.
and the angles goes on the side with tacks.
you know why the 2 bigger piece go in the back because i had no more wood left(to add a whole piece in the back).
besides no one would ever see the back of the stick because it will be screwd to the cabinet.
good luck
