I saw the video on the litter born without front legs and volunteered to build carts for them all. The shelter responded that they had someone local so they thanked me and went with him. The design is very similar to what I was thinking, typically carts are blocks of wood, heavy with wide wheels. The wide wheels creates a high rolling resistance.
I was going to go even lighter using a lexan "frame" and wheels very VERY similar to what he used....
Sure looks like the pup likes it......
(one of the pups had stumps and I was even thinking of prosthetic legs so it could actually walk instead of roll....)