I have a setup similar to what you are looking to do:
My frame is 48" wide by 56" long. The captains box is 24" wide, 13" long. Fits perfect between the thwart hinges inside the tubes. The same dimension for the cooler would be ideal up from too, although I just a cooler I had.
The one thing I don't like about this frame is the oar towers. I may swap them out for the NRS towers to get them outboard a bit more. There are a couple of other designs very similar to this in the raft prone thread, including one with NRS towers I believe.
I just ordered tubing and fittings and built this frame. Not hard at all, just need a saw and some patience...