Based on pictures of existing designs, I started with some sketch drawings to see if it was feasible. I might have a go at drawing up a CAD version, as I'd like to try cutting one out with CNC or even a laser cutter in the future. It would be really interesting to try it with acrylic.
This is my fist Instructable, so constructive criticism and comments are gratefully received. I'd be really interested to see if anyone else has a go at building one.
Step 1: Cut out parts
I used entirely materials I had lying around: some 5mm ply, some 8mm ply and some thin rope; thick shoelaces would probably suffice. As a protoype ply was fine, but I think it would look nicer in plain wood. Wood glue holds it all together, though its annoying to wait for it to set. You'll need minimal tools. I used a jigsaw to cut the wood, but a scroll saw would be better. A drill and bit of similar size to your rope is also necessary.
After a little research online, I started with some scrawly paper sketches to see if the concept would work and work out some sensible dimensions. With a puzzle of this type, accurate dimensions are quite critical. I did make one mistake, but I was able to recover it.
Skip these if you don't follow, it should all become clear in the next couple of steps.