There is a drawing of such a "flopjack" on page 119 in the BCRA book "Caving Practice & Equipment" (1991, Ed. D Judson) - maybe in PDMHS library?
However this is slightly short on detail, so here's a few practical tips.
Use a cylindrical plastic tub. It needs an axle, which is conveniently made out of threaded rod (10 mm to 16 mm diameter, depending on the size of flopjack and therefore the weight of water). Positioning of the axle through the tub is important; it needs to be:
* above the centre of gravity when the tub is empty (so it swings upright after emptying).
* below the centre of gravity when the tub is full (so it tips
rapidly).
The above is achieved by positioning the axle slightly below the halfway position and adding lead weight to the base of the tub. (I generally use a long narrow strip of lead flashing secured around the base of the tub's wall with good quality duct tape - you can of course screw a weight to the base but this creates unnecessary holes which must then be sealed and it means the tub doesn't sit flat on the floor during storage in the garage.)
The axle is held in position through holes in the tub wall with nuts and large washers (on each side of each hole - i.e. you need 4 nuts and 4 large washers). You also need to add 4 rubber discs cut from an old car inner tube and position these under each washer on each side of each hole; this will seal the holes very effectively.
You also need to make a frame; I generally use three pieces of 70 mmx 45 mm wood (the old "3 x 2") fastened together with a pair of coach bolts. This allows the frame to be folded into a tight "Z" shape for transport underground. But you need to add some sort of locking mechanism to make the frame rigid when set up. (I normally have predrilled holes and use an additional wood screw at each joint.) You can of course make the frame from other materials (e.g. Dexion?) but wood is generally durable enough for 2 or 3 years continuous use at least and is easy to work with.
For "bearings" I drill a large hole through the base of each leg of the frame and stuff a piece of plastic pipe through in which the threaded rod axle sits loosely. There are various ways to secure this plastic pipe in position, from just an interference fit (not recommended) to inner tube loops stretched over it and doubled a few times, to cementing in place to more complex screwed devices. Because the frame can be articulated it can be assembled around the axle onsite then locked (with the wood screws) such that the axle cannot then escape during the hundreds of reciprocations which you hope will happen. The plastic pipe "bearings" should be a significantly larger diameter than the axle to minimise the possibility of it becoming jammed in muddy conditions and to ease assembly.
The frame is hung from two attachment points in the cave roof (or on the walls) by tying it up with bits of polypropylene rope (I generally use the 5 mm or 6 mm stuff - or thicker if the flopjack is an unusually big one). Use knots which are easy to untie to allow fine adjustment, to achieve optimum positioning.
I think that's about it. Such things tend to benefit from a low tech approach as this seems to be more durable in a cave / mine environment and it means that if trouble shooting becomes necessary it's often easier to bodge up a low tech device.
It's best to do some trials with the device at home before deployment so you're sure it works. This can provide hours of entertainment - well, for the neighbours at least!
Enjoy . . . . . . .