No need for extra support as its solid enough, and only needs to hold the weight of the leds, your mothers chooks could roost on it and it wont move.
I just place the pcb on the base board with the paper artwork on top and a sheet of plain old glass on top to hold it flat...........works well for me.
Keep it simple, and it will work well, as long as the height dont change and the exposure time remains the same everything will work out every time.
One thing i often notice is how everyone puts so much importance into how good their light box is and how much they paid for it and bla, bla bla.............
To the point it puts others off at having a go at making their own boards, as it all looks too hard and costs to much
And here i use $20.00 of leds and some discarded junk, with very good results, and more than likely make more boards than the guys with the high priced light boxes.
Its easy have a go i say.......you wont regret it.
Pete.