My o-ring is broken (too). The battery gets wet in the rain and the power meter stops working. O-rings with these dimensions (22 x 1 mm) are almost impossible to find and it seems the maintenance kit won't be available in the foreseeable future.
I used a 10 cm piece of 12 mm teflon tape. Sliced off 1/3 lengthwise with a ruler and box cutter, and rolled it into a tiny rope. I pulled it tight into the groove of the cap with a half knot. Then cut off the excess lengths. Problem solved.
