Making of durable plywood frame
Of all your designs, the last one certainly has much to its credit. If you use sheets of plywood to create the individual inverted-U shapes, you'll gain that much more strength. Consider a combination of the design previous (with the gussets) and cut your shapes to include an arc to the inside of each leg. This provides for contiguous transfer of forces as well as looking good.

Alignment is unlikely to be critical in your application, but you'd want to have as clean alignment as possible. Drilling holes in each plate in the same location, then pinning the layers with a dowel rod will make for ease of assembly. Once glued together, you can glue the dowels in place or remove them as you like. 

If your plywood source is recycled skateboard planks, your work will resemble the last image that much more!