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find a metal bottle cage
find or cut 2 pieces of metal sheets, first should be able to fit behind the rails, and then second should be of the same measurement
find 1 bolt for centre hole and 2 bolts for far end holes.
on these sheets, drill a hole in the centre and two at each end using a drill
the diameter of which is equal to the diameter of the bolts you find in your home.install one sheet behind the rails, then insert bolt in the centre hole. then, screw on one nut and tighten.
attach second rail, then insert bolts on far ends. then, screw on nuts and tighten.
attach the bottle cage on centre bolt, then screw on the nut and tighten.
wrap small pieces of ductape on excess thread on the bolts in order to stop nuts from unscrewing
i have not given exact measurements of the bolts because you can use anything for far end bolts. However, the diameter of the centre hole bolt should be equal to the diameter of the hole on the bottle cage.
Lastly, the sheet you put behind the rail will not go in(depending on the saddle shape, mine did not), so you will need to bend it slightly. That is ok because it will unbend as you install the bottle cage.
no, this is homemade.
you can ride on the roughest terrain; jump on curbs, over potholes; wiggle; fall; the bottle will not move even a mm. that's how stalwart it is.