you can just buy a really large allen-head bolt and weld a bolt upside-down inside the head. then you just screw a nut onto the bolt and tighten the seats in like that. once they're in, you remove the nut and those things aren't going anywhere unless the thief just so happen to have the correct size nut and wrench to get the bolt out.
and to remove them yourself, you just need two of the same nut. screw them both part way onto the upside-down bolt, back the bottom one off until it tightens up against the top one, then you have something to grab onto to remove the seats. total, it'll cost you less than five bucks for both of your seats.