1: wet up part with carbon and epoxy, then wrap peel ply and paper towels. Place this into a waterproof bag, then sink into a deep bucket of warm water - say, 1m deep.
2: wet up part with carbon and epoxy, then wrap peel ply and paper towels. Place this into a plastic bag, put in pit, fill pit with sand.
1: 1m sink in water equals 10% of vacuum (atmospheric pressure). It is not enough, you'd several meters deep pool to have enough pressure.
2: atmospheric pressure equals 10m of hydrostatic (water) pressure. Sand weighs not even twice as much as water, so you'd need several meters of sand piled on your part to have comparable results.
It should be remembered full vacuum produces not at all very high pressure, for example board industry uses hydraulic presses with several times more force. But for most uses vacuum works well.
There is no reason to try reinvent wheel. Get somewhere working vacuum pump and use it. Fridge compressor works very well, it can pull at least 90% vacuum with no problem.
Sand box could be useful for small parts if after burying object there some plate is put on top of it and then additional pressure is applied somehow (clamps, car jack).