What i have done years ago is plastic-weld three water barrels on top of each other to form one huge but tall barrel- the outlet being the one at the bottom. As the barrel is full the pressure is greatest and as it depletes the pressure becomes less. Weight was not a problem as it was on a concrete floor and the barrels were strapped securely to the garage. To increase the force of water I took some copper tubing and squashed one end flat but not entirely closed (paper thickness). The other end I used a worm-screw/jubilee clip to secure the copper pipe to the hosepipe.