Ok a different thought .. kind of like #29..
use a hollow tube seatpost, run a cable loop out of the top of the seat post around the seat rails and back into the seat post , then run the 2 lengths of cable down the seat post tube,
and perhaps fix them to a stopper in the end of the seat post..

Or,with the other end secured in a plug that with enough cable length, slides down past the water bottle braze ons,
inside the seat tube , then you can fit a longer bolt in the bottle cage, to keep it from pulling back out out. ..

that should baffle most people with only enough brain power to think of quick grabs.

there are security allen bolts which in the center of the hex socket there is a pin it keeps the common wrench
from going in .. the hex key that goes with it has a matching hole in it's end.

that tamper proof site lacks allen capscrews.. I note. just metric button and flathead.

I use the Bladder bag out of a 5Liter Vin Ordinaire box as my winter saddle cover..

it, not only keeps the seat dry, it makes a saddle seem pretty ordinary too.