Here’s a schematical drawing of the device that should explain it all:
I have made a cheap device and successfully tested it in blackpowder preparation (precipitation method).
Results are comparable with vacuum filtration (heh, why shouldn't they).
Here’s what I used as a container. I've specially chose one, made of polypropylene, which can handle most common solvents (acetone being one of them). That cup is originally used as some drinking bottle:
I have drilled many 1,5 mm holes in the bottom of the cup (every 5 mm). An air input was made in the lid, by attaching a metallic pipe through a rubber gasket (gasket penetrates the lid). A bicycle valve was attached to the air input, by improvised means. This design became a bit different than on schematic pic above, though that’s just to make things more convenient. Below is the result:
As you can see, instead of hose clamps, I used simple copper wire.. really low budget, but that does the job too.
Here you can see the device screwed together:
To filter, one should place substance that needs to be filtered on a piece of textile, form a baggie and then place it inside the container. Whole mass should be pressed slightly to remove air pockets. After, lid is closed and air is pumped with bicycle or car pump (through the valve). The water will be removed fast, nearly as good as with vacuum filtration.
However, make sure not to over pressurise the container.
On further notes, it's better to filter larger amounts of substance at once. During filtration device must be held with filter down.. (duh!)