Bending 2x4s

put each end of the 2x4 on bricks or something so the centre is raised off the ground, then take some old towels or something that holds water well and drape them over where you want to bend it. soak the towels. put a hefty weight on top of where you want to bend it and make sure to keep the towels wet. if you'll be gone for a while or for overnight maybe a tarp over it to keep the humidity in. maybe 2-3 days would do it for best results.

soak it then follow up with a heat gun, it is the heat that will make the wood really bend. Really soak the board, best to submerge it for a day or two and it will be a lot easier to bend.

Either use a 4x4 or 2 2x4's on the base as you won't want the top board creating a warp for the bottom. To hold the curve you want your vertical supports to vary in length obviously so the middle board is the shortest, figure it out however you want. Screw it all together and you're good to go.

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A friend of mine already has one made, I was hitting it this weekend. You coukd message john18061806 for a possible pic

instead of a 24 try using Trex, its a synthetic wood for outdoor decks, it might bend better, and will be more durable when its wet because it doesn't absorb water.