Great idea and good job, well executed!
I was looking for some legs or a holder for mine too, and really like your 3d print project (on my unit, I currently made some legs from wood and spray painted them black, just to have some space at the bottom of it, you can check it here Extra cooling on FW Desktop )
But I have a question, regarding the heatsink, how is it mounted into the frame?
And how well does it work without any thermal pad? Did you measure any temperatures to have an idea of before and after?
Also, regarding the air filter, you could add some thin double-sided tape around it and mount it like that.
And PS, which gap are you talking about?