Regarding the construction, I think you could probably do what you suggest. It seems important also to build the two corner panels to tie it all together although a few builders seem to have omitted them. If I build the boxes out of MDF I may well build the top, bottom and corner panels as a separate sub-assembly, because I'd want the box joints to be mitered (more difficult for the top and bottom panels. Given the overall weight, there would seem to be merit in making the system in a number of separable parts, so long as you have a robust way of jointing them so they can't buzz against each other. I was also thinking of using ply for the woofer panel (the one that faces backwards into the corner) so I have a better material to attach the woofer to.My current plan is to paint the boxes, except for the front of the woofer which I thought I might veneer for contrast. I thought they might look a bit smaller that way as well...
I haven't heard the drivers yet, but I'm keen to have very clean bass, so I expect to buy the JBL.
tim