I know this isn't a home made one, but I use this "magnetic folder" loads. It normally clamps the metal via an electronic magnet to hold it in place as you fold. Unfortunately the magnet shorted out but I still use it by clamping with caver clamps.
It came with a selection of top plates, but of course you can clamp anything on top to fold round. That way you can clamp tube, for instance, to get a round fold.
I'm sure the design would be easy to copy for a home made equivalent.
Of course, for bigger jobs I have this monster.....
Currently using it to fold a rolled edge .