Cut the plywood into two semi-circles to form the frame of your bench. The semi-circle should measure about 1-1/2 feet high at the highest point and 5 feet long. It is preferable to shape the plywood into a half-heart shape rather than a completely round semi-circle. By tapering one end, you will give yourself the traditional backbend bench shape.
Sand any rough edges on your plywood slats. Remember: you will be laying on this device, so you should aim to round any corners and remove all splintered wood.
Fix the first slat into place to one side of the bench. The slats will form the actual bridge on which you will lay, so it is important to fix the slats firmly. Do this by applying wood glue then using a nail gun to hold the slat in place. Do this with the first three to five slats, following the curve of the semi-circle, and then fix the slats to the other side of your bench.
Continue fixing slats along the bench at even distance from each other until you have formed a bridge. Slats should be close enough together to support your body. However, leaving a 2 to 3-inch gap in between slats will allow you to form the round bench with rigid, flat slats. It is the gap between the slats that allows for the curves to happen seamlessly.
Clamp the final product together once the slats are in place using a vise. Place the vise perpendicular to the bench, anchoring the plywood on either side to the slats. If possible, place one clamp on either side of the middle of the arch. This will help the wood glue to seal evenly throughout the bench. Leave the bench in the clamp for at least 24 hours to allow the wood glue to dry appropriately.