All you need is one 23/32 inch plywood sheet ($30), one 8 ft 2x4 ($5), a box of screws ($5), a saw, and a drill. The three heights are 30 in., 24 in., or 19.5 in., and you can do multiple upper, lower, and full body exercises with one piece of equipment for a little time and ~$40.

All you need is a piece of 18x18 piece of plywood, half inch floor flange, half inch cap, and half inch by 4 inch pipe nipple to accelerate your plank workout.

All you need is a pole or stick, a thin and flexible rope, a carabiner, and weight plates!

Add a wheel to make it even harder!