I think that the momentum of the sloshing of the fluid would have that cradle constantly swinging. I'm skeptical it would work well.
I've thought about stripping the parts off of the Fimco and attaching the stuff to a homemade tank made from a plastic barrel. I'm not sure if there are any 25 gallon sized barrels out there, but I'm not sure size really matters. I do have some smaller barrels that I think would work okay but would need refilled more often. I know they come in 55 gal size but I'd rater use something smaller. I don't use mine on an ATV, I have it mounted on a 3 point carry-all, so I suppose a 55 gal would work with my tractor.
A couple thoughts on using a barrel...
If the barrel is laid on it's side, the hillside spraying issue would still happen but on a much lesser degree than with a flat-bottom tank like Fimco uses. But the barrel could be positioned upright and possibly tilted slightly in a cradle with the intake located appropriately, which would allow the majority of the fluid to be sucked out.
One thing is for sure though...a homemade barrel style tank would be much faster to flush-out than that stupid, idiotic, ridiculous, dumb Fimco tank with the garden hose sized drain on it. Barrel bungs are normally 2 to 3 inches in diameter.
For the life of me, I can't understand what kind of idiot engineer would design a drain hole to be only garden hose size. Takes forever to drain, flush, drain, flush, etc until the tank is clean.
If I owned the Fimco company, I would redesign that ridiculous tank in a heartbeat. I hate stupid stuff.