Re: Swiss Ball GHD
I made something similar to what is shown in the link in Skylar Cook's post above. I'll call it a "GHD piece" - this gets put on top of what I use for box jumps (basically something like a sawhorse, but wider on top), and I hook my feet under a metal pole clamped to my squat rack (I've taped some foam around that where you hook your feet). The "GHD piece" that I made is not as tall as shown in the link - it is basically just the curved part (and more of an oval shape, rather than a semi-circle) without the straight sides underneath. I think this makes it more stable. I find that this stays on the box by itself without needing any sort of attachment.
The "GHD piece" is a number of sections of scrap plywood cut into "D" shaped sections with a jigsaw. Precision in the cuts isn't important as after gluing together, you cover with thick foam rubber, cover that over with vinyl and staple that to the wood. Dimensions rougly 12 inches (wide) x 10 inches (deep) x 6 inches (high). Works fine for a home gym, and takes up no more space than is already taken up by the box-jump box.