I put together a simple helmet lock that may be of interest. It's made of a loop of lamp cord wire with the ends soldered together.
To use it, you loop it through the helmet D-rings, and loop the two ends around the frame bolt at the front of the seat. Then reattach the seat.
The wire loop is hidden so it can't be cut.
Start with a 13" long wire, strip the ends back 1/2", twist them together and solder, include shrink tube if you want.
