I don't think my Dive-Kart fits your needs, but maybe you can pick & pull an idea from it to help your design.
Mine holds over 400lbs of crap and is rated for 700lbs. I can do my 1500ft walk in a single trip with locking the kart at dock. It holds:
50lbs of ice cubes for lobster & lionfish
Bug Bag
12lb no crush Rob Allen foam dive flag float
12 pack bottled water
Back Pack no-flame chemical oven for 6 steamy 400 degree individual lunches (gets hangry from finning hard diving)
2 LP121 water-heater sized steel tanks & 1 LP108 for a total of 405 cubic feet of nitrox (My sac rate ain't low)
2 gallon sized 'save-a-dive' dry bag for gps, phone, opticals, sting-kit, gopro, zipties, baggies, wrenches, etc, etc., etc.
Full set regs/fins/BPW/neoprenes, etc
1 Koah (swiss army knife style) speargun with looper & lionfish spear
The base cart is a nylon-glass Harper truck convertible hand cart so it won't rust. The lift plate is cut back to only 1inch so the entire kart breaks down to only 12 Inches thick by 43 inched tall and fits on the floor behind my driver's seat and the back bench seat of my car.