I have  made several DIY carts over the years depending on the kayak, usage, etc. Of all materials I have used, PVC and an aluminum shaft has proven the most durable and versatile with the added benefit of ease of assembly/dissasembly if you don't glue the PVC. Not pretty but it is cheap and works. The picture below is used on my Ocean Kayak Scrambler and fits on the scupper holes.