As steel has gong insane in cost .looking at some of the Harbor Freight frame and alike . how about pallet rack beam .Can hold many tons of weight and because of the folds makes it super strong and lighter than tube .I .Seen the price about $5.00 a stick. Cutting the corners at a 45 would help the outside lineup on inside rails may need to add a bit of blocking. a few more spot welds on the seam . then run some sealant and keep the seam down for drainage or keep the seam up and covered. Where the axle ties in add a bit of 1/8 '' strapping .it is in the shape of a tube and with the folds super rigid . I believe it makes % 50 stronger than tube for weight