I think this problem can be easily and cheaply solved by a simple modification of the position of the bars that support the boat. Of course, my preferred solution is to get a slimmer, higher performance kayak that will fit better.
As another user pointed out, there is too little space between the tires' air valves and the wheel hubs to use a bicycle floor pump or my other two hand pumps. A service station air hose might fit, but it would probably explode the tires. With some wiggling, I can use my bicycle's mini-pump, but then lots of air leaks out when I struggle to disconnect it. The obvious fixes would be to have the valve stick out at a slight angle, or to use a tube with a shorter valve or a wheel with more clearance.
Now that kayaking season is almost over, I'll have time to try to figure out a way to use the PaddleCart with my tubby boat. It's just too nice a piece to get rid of without trying to make it work for me.
I have encountered two problems. I thought the lack of a "kickstand" would be no problem, but now I wish it had one. This is just a slight inconvenience. The second problem is greater. The air valve on the tires cannot be connected to by a regular tire pump. The air valve comes straight up in the middle of the wheel - it needs to come out at an angle. I contacted PaddleCart about these two problems, but got no resolution. A neighbor has a PaddleBoy cart with pneumatic wheels, and I think I would be happier with it.
The wheels are also a little suspect. The exact wheels (found elsewhere on the internet) have a weight rating of 100 lbs each. Yet they claim a static weight test of 300 lbs on their site. We had my friend's loaded to about 250 and it worked well, but the axle bent slightly.
As negative as this sounds, if I can't get into a friend's shop to make a similar design, I'm buying one of these. It lasted well, while several other carts on our kayak/camping trip failed catastrophically. Clean up the fabrication and it would get 11 out of 10.