I've owned three and carts three kayaks
My first cart was one of these
My second cart was made of PVC, I don't have a pic of it.
My third cart was a Wheelez cart with the big sand wheels
I sold traded my first cart for a stick of gum
My second cart was traded for the wrapper that holds the stick of gum
while I agree that you will you get a feeling of self accomplishment if you make your own cart you will still have to deal with the fact that the dolley wheels DO NOT work in the sand, I am only speaking from experience and nothing else. Take my advise or not but I can assure you that the first time you have to pull your kayak through soft sand you will remember everything I said.......I guarantee this.