Just finished a rear speaker project
I just finished making my own enclosure and installing the speakers last weekend... I will try to get some pics up here soon.
I have a '77 and just made a square box out of 3/4" MDF. My carpentry skills are horrendous, but i think the dimensions were 36"W x 15"H x 7"D. I haven't seen if the T-tops will fit yet, but I'm pretty sure they will. I also covered the box w/ vinyl from a fabrics store using a spray adhesive. The setup is 1 12" subwoofer and 2 6" speakers powered from a 4 channel 1600W amp that I have bridge to 3 channels. I also removed the "shelves" of the 3 door rear compartment and put the amplifier back there, so it is out of sight, as well as all the wires.
All in all, the MDF was ~$35, throw in another $20 for screws and glue, and another $10 for the vinyl, and w/ a little determination you can get a pretty decent box for well under $100.