When i built my open backed 2x12, i used 18mm ply for the outside (straight from B&Q), and some 15mm(i think) MDF for the baffle. Inside, i used batons that i screwed into the sides, as well as glued, so it's very strong. Don't go for any weird joints- i tried to do a mitre joint, but my circular saw was too inaccurate, so i had to fill a few gaps. When you go to paint it, i'd suggest doing a few layers of primer beforehand, otherwise the paint will take on the grain pattern of the ply, and you'll have issues with the edges where the pain will either not take very well, or just keeps soaking in. If you buy a 4'x8' sheet of ply, you'll have enough (if you're clever with the layout), for 3 1x12 cabs, or perhaps 2 if you use the ply for the baffle also.
Enjoy :)