Finally got around to seeing how PBBx components would scale for a baseball cap. Worn backwards, would it be a little less conspicuous? I wanted a rig I could use hands-free and hear what the mics are "hearing" without needing headphones. The stereo imagery should have impressive depth and reach. Recordings should play well on speakers as well as headphones. There are only a few small compromises in quality as best as I can tell.
In solving the structural integrity aspects, make sure that the leading edges of the boundaries end where shown. Add no hard, reflective surfaces in between them, even behind the foam. I'd consider adding a "V" wedges behind the boundaries that attach to the cap sides. The angles can flex and change a bit, no problem. You might also want to clip off any part of the brim that sticks out beyond the foam baffle that is under the capsules-- that would create another boundary. Off-set center of mic capsules in the vertical boundaries .75" from top on one channel and .75 from bottom on the other. For wind-screening, I'd start with stretchable fabric like a sheer Lycra? pulled across all of the mic components. No obstacles allowed on the boundaries themselves. Make sure the diaphragms of the mics are flush with the boundaries.