There is a 3x7 plate bolted to each of the rear bumper bracket locations with 1/2 inch bolts. They are welded to the end of the L shaped brackets hanging from the pivot arm in the first picture. On the front I bolted two 4x8 plates to the side of the frame rails at the front bumper bracket locations. The side of the L brackets was welded to these.
It was more than a couple hundred bucks. The price of steel is pretty high. Everything was 3/16 wall thickness. I bought 60 ft of 2x2, 20 ft of 2.5x2.5, 3 ft of 3 in pipe and 3 ft of 3.5 in pipe for $305. Locking 650 lb casters were $100, jacks were $55, a couple pounds of grade 5 hardware and 5 lbs of welding rod comes out to around $475. Still not too bad considering what some vendors are selling them for.