the Sporting Gun or Shooting sports had a photo of a chap who had mahe is own using some box section steel and two wheelbarrow wheels. It then became the back to his hide. Failing that motorised golf trollies are a good starting point and build to them. I use a manual trolley (one without a motor) with a crate attached to it and plenty of bungee cords. The only thing I carry is my gun, I attach the trolley to a balt and just walk. The wide wheels mean it doesnt sink. Bear in mind I use crutches all of the time so it has to be easy. Good luck, and DONT give up