Take a look at some commercial construction sites in and around the area. I found mine as a piece of scrap that they let me haul away for free. 18"x 3/4" thick sewer pvc. I cut 2 feet off the ends, cut them in half to make 4 hull cradles. Two painted plywood cradles bolted to the trailer and end caps. Maybe I put $50 and 8 hours work into it. That was 8-9 years ago and it's still holding up. I'm 228-lbs and can stand on one of the four cradles without it breaking. I had posted pictures of the rig here once before.

Just remember to take BOTH end caps off the tube when you launch the boat off a ramp and sink the trailer. Especially if your end caps are sealed. If you leave the front one on, that much air volume can and will float the trailer, boat, rig and all. How do I know?