Box keepers are VERY hard to adjust with some minis. After having one box keeper bridle, I will only have open keepers now. Box keepers are cheaper to construct, though, which is why you see them so often on mini harnesses.
I talked with David Freedman about what part of the harness he felt needed to fit absolutely perfectly. I had assumed he would say the saddle, but alas he immediately stated the bridle.
When I ordered my show bridle, I took apart one of my better fitting bridles, measured each piece, and then asked for more or less material in certain places. I am pretty happy with how that bridle fits, although I would still love to tweak it in a couple of places. Yes, taking actual measurements off the horse has resulted in bridles not fitting as well I needed. Fitting bridles properly is an art (see above paragraph) that requires time, talent, and treasure.
Yes, the caveson needs its own hangers, with keepers that attach to the cheek pieces so the blinkers don't gape. That is the real reason for a caveson on a driving bridle.
Myrna