This shows some decks made with Poly Max and tubing support-
Here's the frame with seat, oarstands, icechest, etc.— pretty standard stuff. The unusual bit is a 3-piece dog deck, of PolyMax kennel floor, that splits to create footroom for a human (or at least humanoid) passenger.
The separate frame up front was built to fit York Packs, which nestle down nicely between the tubes, a high and buoyant load that helps carry the bow over waves. If I need weight up front for good trim, I carry 3-gal. watercans outboard. Under the dog-deck and seat I load the tent, grill, and similar low-profile, squashable stuff. The two aluminum goodies by the seat are Personal Dry Boxes from Cambridge Welding (from Cascade Outfitters). When the frame is rigged, the tubes tip them outboard, so you can flop the lids out and use them for trays.

This is the deck set up for the dog. In use, it'll be held by 1-ft. straps.

The center section has been taken out and the two sides pushed apart so someone can ride on the icechest (the plastic buckles are to mount a Crazy Creek canoe chair).

This shows the deck from underneath. The inner edges of the wing pieces are supported by aluminum tubing cut to rest on the crossbars and pop- riveted to the plastic grid. The center section is supported by a piece of doorsill from the scrap bin, held on with cable ties. It goes farther forward than the other pieces, and the seatboard rests on it. I miscalculated a bit— cut the frame tubes too short— but it's simpler to trim the decking than re-cut and re-drill new frame tubing.

Here's the way I cut the end of the aluminum tubing to fit over the crossbar. Tubing is quite stiff for the weight (at 225, I can stand on this deck but probably not bounce on it) but you could use angle or channel instead. My choice of material often owes to the fact that I live 30+ miles from the nearest hardware store.
I'm looking forward to a shakedown trip this fall. Thoughts and suggestions gratefully accepted.