I haven't built it YET but here are some ideas for the group to play with.
The marineland GV is cheap ~ $7 + - IIRC ( just a hose and funnel tube ). The penguin power heads are cheap @ wally world ~ $16 for a 175 GPH job.
Simply stick the end of the hose onto the intake of the power head and vac ! You'll probally have to drill holes into the upatke funnel, perhaps a couple of rows spaced evenly around the circumference of the tube to keep the suction down. I could see a short length of larger diameter clear hose that will fit snuggly over the vac tube that you can moved to cover more or less rows of holes until the vac suction is just right then glued in place. We'll most likely need a dirt trap perhaps a large ( wide ) pill bottle with some filter material stuffed into it in the line after the vac funnel tube. Better yet stuff the vac funnel tube with the filter material and ditch the pill bottle
...Again I haven't made this yet, just throwing this out there for the tinkerers ...
The store bought power gravel vacs go for $70 + at least the ONE i saw did.