My bandsaw is really quite rubbish and the machine itself cost less than a commercial jig would, and probably less than the bits for a complicated diy jig from a plan, so I decided to have a go at making a jig myself keeping it as simple and cheap as possible, by using scrap wood I already had, and a plan from my own brain.
I cut a piece of 12mm ply and drilled a hole in the centre of it using a drill press to ensure it was exactly square with the ply. Then I attached to cut offs from another project which I knew to be the same height.
I screwed a decking screw into the drilled hole to cut a thread in it, then removed it and ground the threads off the tip to make it into a point, then re-inserted it into the hole.
The jig works by clamping it to the bandsaw table, with the screw point in line with the with the blade and the distance from the blade as the circles radius. The piece to be cut slides under it and is held in place by screwing the screw down until it pins the workpiece in place, the point embeds into it and provides a pivot point, either in the piece itself or a sacrificial scrap piece stuck to it with double sided tape.
It worked pretty well and even worked with the table tilted to cut some pieces for a small segmented bowl, I found that screwing the pin down too hard held the piece to the table too tightly making it hard to turn the piece, but screwing down into the piece to punch into it then backing the screw out a fraction of a turn worked really well. The photo shows it cutting a piece of maple at an angle for the bowl.