You are right about trying to flush the piece, it's impossible since all helmets are created differently. I think as long as there is enough material thatit can be shaved down w/o compromising the integrity of the unit, then it is fine. If someone wants to get it to sit tight, they can get out the trimming tools.
Price range is good.
Connection... It should either be a 9v connector, since all the AA/AAA battery boxes I have seen have the 9v connector on them, and people could just use a 9v as well if they wanted. Or it can be bare wires, people pick their own connector. It depends on the level of DIY and price tag on the finished product.
Also, you need to get them in a finished faceplate for final photos before going to open market, well or rely on one of us early adopters to take a shot! IT will help with the finished look of the product, for sure.