One of my ongoing projects that's been put on the back burner for a while is making a 'drop out' box, specifically for the circular saw. The idea of capturing virtually all the sawdust is appealing, especially if it means that I don't have to clean out the paper filters in the vac.
Cyclones do the same sort of job, but are costly and difficult to make...this sort of approach does the same sort of job, can be built for a fraction of the cost, and is slightly easier to construct.
If you're unfamiliar with the way these devices work, it's simply a box, made from whatever gash sheet materials are loafing around in the 'shop. The dust is drawn in by the suction of the 'shop vac and is directed around the interior by cunningly placed baffles, such that the air flow is slowed down and in doing so, deposits the sawdust at the bottom of the box, or in a suitable container. If the box is made correctly (ie there's a long enough passage for the air and it's slowed down sufficiently) then in theory, all the dust is captured and nothing escapes into the vac. The critical thing is that the box needs to be sealed effectively, so there's no loss of suction.
The hold up for me, apart from the lack of heat in the 'shop, is sorting out the pipe work, but I have an idea that by using plastic rainwater down pipe I ought to be able to rig something up.
That's the theory...practise might be different.