I finally got around to making the gaskets I mentioned above. I started with paper, laying it down on the parts and marking out a pattern. This took several tries of scanning the hand-formed pieces, tracing them, printing, cutting out, adjusting, etc. Then I bought some 1/64" thick rubber sheeting from eBay ($10), traced around the final pattern in pencil on the rubber, and cut the shape with scissors. The holes I cut by hand with an exacto knife in situ (i.e., be warned, the holes on the below links are not super well positioned, also the pattern is missing the round bolt hole at the bottom). Also, probably worth clarifying that there's just one pattern — the two sides are symemtrical.
Overall, these fit fairly well. To get them in position I first lay them down, averaging the "offness" of the pattern until I have it positioned right. The pattern isn't perfect, but it's close enough to satisfy me, with a fairly obsessive and neatness-oriented personality, for a part that is well out of sight.
Here's the deal with the surface between the plastics: It's a rather large, flat surface area, with a thin vertical "lip" along the edge where the middle and side plastics meet (see diagram above). The rubber should creep up that edge ever so slightly. This way it's barely visible when assembled, but it creates just enough of a gap to keep these pieces from touching. I use scotch double-sided tape underneath it to tack it down. That stuff is super thin, basically a strip of adhesive with a little film in between.
Here's the pattern as a .PNG picture file. You'll notice that the pattern is cut in two, that there's a 1" square for size reference and also targets to show you where the two halves of the pattern line up when you cut them out. This is so you can print it on a letter-size paper, cut these out, line em up (use a light, or the "quick flip" method where you lay one page over the other and pull up the corner of the top layer repeatedly to compare the position with the one underneath). And need I mention… Cut the pattern out of paper and fit it up before cutting your material. Once you've bent this first paper pattern all up doing this check, make a new one and use that to trace onto the rubber.
And for the more involved crowd, here's the same pattern as an .EPS file (created in Illustrator, can open in a range of graphics programs).
Let me know if you laser cut this, I'd be interested to hear about that.
The end result (for me at least) is a very rattle-resistant set of front plastics. If you're getting some buzzing and you're wondering whether this approach is the one for you, I would suggest figuring out a way to ascertain whether this is the issue before messing with the work. You can identify a condition that reliably elicits the buzz (e.g., a certain speed, RPM, etc.), then jam some paper down in the seam without any disassembly, invoke the same condition and see if the sound has disappeared.