I built mine with a non pressurized ballast tank pump system that works great and cost 40.00$ bucks.
The internal frame was cut in two and the 4 inch dia . tank glued in slightly off center to the verticle so that the pump inlet/exit tube fits underneath.See my build in the builders thread.
The nice thing about this system is that the cg does not change as the tank fills or empties,unlike the Engel pump .Very preciese control of the dive or serfaceing operation is achieved as well as hovering.
See system diagram for general layout. Also the Engel pump has a long screw jack that has to be acomidated and takes up valuble space.
The tank also has baffles.
This dive system works very well ,is cheap ,parts readily available and there is no limit to the size of the tank to obtain a correct water level line.
Check out my latest pics.