the second one
both were exercise bike seats where i got rid of most or all of the framework and made my own. i guess you could do the same with a bobber seat if you wanted that exact shape. i have to say though, my seatpost being angled back so far makes this easier (the second was really for a more upright seatpost but i couldn't work it out). you'd probaly need a much shorter spring like off a kids bike or something.