Interesting topic. I wonder if you could use the 1/2" board just for the baffle, then use a 2x2 frame but just have 1/8 or 1/4" ply for the rest of the cabinet... Maybe differnt speaker configurations would be lighter: 1x12 and 3x8, Or 2x10 and 2x8...
Regardless of the cabinet maker/style you choose, I think the total weight is very dependent on the speakers you load it with. I have an open back 4x10 that I had ceramic Eminence speakers in (heavy!) and swapped them out for alnicos- the cabinet is now a very light one hand carry.
I made one by copying the dimensions of a Super Reverb cab, putting 4 Kendrick Blackframe speakers in it, but the real secret of it being so light is that I made it from real old pine boards I found in a salvage place. The wood was so old it was feather light