Don't buy fancy solid timber, no sound quality benefit. Use 3/4" (19mm) good quality, void free, birch ply. If you have the means to cut the edges dead square, glue and screwed butt joints are just as good as fancy "finger" joints. If you can't cut the panels accurately yourself most DIY stores can do it for you but get your dimms' right!
Otherwise use 25x25mm rails and fill the gaps!
Since it is a self build you can experiment. Sneak the dimensions of some 2x12s and then go ~5% bigger, you can always make a cab acoustically "smaller" by putting in some blocks of wood or expanded polystyrene.
Make the back panel in 3 equal sections then you can chose open or closed back.
Make the front to back dimm' about 2"(50mm) oversized and set the baffle back an inch+. Then front load the speakers and make a sub frame from 1/4" ply, cover that with fret and fix with Velcro. Front loading the drive units is "safer" since they can be fitted last thing and are of course very much easier to swap out for comparisons. Celestions come with a rear gasket kit.
2X 16 Ohms ? Fit a jack per speaker then a nice chunky 5Amp switch to parallel them (unless you want a dedicated 8R speaker)
Dave.
