John, it should be a pretty simple build and attachments are readily available. I used a less powerful vacuum to blow air down and more powerful vacuums to suck the dust out through the top. My thought is that if something heavier than dust comes off a model then it might be heavy enough to fall down while the dust can be light enough to be sucked out of the top. If the vacuums are on the bottom then the heavier items might go down the tubes just as easily as dust due to gravity. I've used 2 vacuums to suck the dust out but someone told me there is a V attachment for the vacuum so I could have used one vacuum to suck the air out. Having two has come in handy for different purposes, anyway.
TNDENTRR, Compressed air is great but the purpose of the vacuum box is to make it so the dust does not escape back into the room and simply settle on the trains again.