This is Thomas, the guy behind the DIY filter site.
And that's an excellent question! Does it work? I'm a believer in open data (and a nerd), so I've done many tests of the filter with a particle counter. You can see the data:
1. You can see a video of a test of the air coming out of the DIY filter here:
http://particlecounting.tumblr.com/post/55491154130/heres-test-1-is-the-air-thats-coming-out-of-the
2. I've done tests of ambient room air before and after turning the DIY filter on. You can see my data here:
http://particlecounting.tumblr.com/post/55665836265/purifier-tests
and here:
http://particlecounting.tumblr.com/post/55947941919/diy-compared-to-expensive-filters
3. And you can come to our workshops and test the DIY filters live on the spot with the particle counter.
http://smartairfilters.com/#workshops
As far as the seal goes, ours get a pretty good seal around the filter. For an improved seal, you can try using the more flexible pre-filter that comes with some filters. Perhaps your HEPA was different from the ones we use? Nowadays, I've been using velcro straps, which gets a tighter seal. Pictures here:
http://smartairfilters.com/#steps
Finally, this is a bit of a philosophical point, but how much the seal matters depends on whether you're (1) sending out air from a very dirty source into a clean room, as in a vacuum cleaner or (2) pushing air that's already in your room through a filter. In case 1, the seal is very important. In the second case, the seal is important, but a less-than-100% seal does not render it "useless," as you suggest. All you need is for some air to be going through the filter for it to be removing particulates from the air. But you don't have to take my word for it. Doctors at the University of Michigan have tests with a similar set up, and you can see their seal is not perfect:
http://particlecounting.tumblr.com/post/56577062878/independent-confirmation-ive-always-suspected
Thomas Talhelm
http://particlecounting.tumblr.com/