I never thought about spacing the ballast away from the fixture with spacers, that sounds like a good idea. Is there any concern about the lack of heatsinking by not having the ballast against the fixture body? Or doesn't the fixture provide any heatsinking?

The fixture does provide heatsinking, but most ballasts are not supposed to overheat anyway. A good measure can be to attach a heatsink to the ballast, which is made of metal but not magnetic like aluminum plate (can be cut from window frame beams etc)