1 mil of kapton, so long as there are no sharp protruding ends sticking out, will withstand hundreds of volts. But in general, it is a poor engineering design to do so. why not add another board layer and be done with it?
I suppose if you were stuck, you could add a non-conductive coating to what the board is mounting too, in addition to the kapton. Mabe anodize the aluminum surface, etc.
5 mil washers will do NOTHING if the board flexes under vibration or if the board warps.