Zip ties
We don't usually recommend them because they degrade in ambient ultraviolet light ofer time and will eventually fail. But if you can get wide ones, and if you can get them tight enough (they need to be VERY tight) then for short term use where they'll be replaced frequently, you're probably okay.
Try putting a small kitchen knob on the seat screws. You might need to change the screws to 1/4 inch to match the knob's thread, and make the screws a little longer to bite well.