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The following are observations from installing a Canadian Kenmore 50135 - aka 175.50135, made by Insikerator - 06/28/11.
The secret seems to be to exactly center the unit, so that the rubber flange at the top of the unit appears to have the same width all around the drain hole, when observed from above - look for narrow points in the grey flange around the edge.
This is really a two-man job, with one supporting the unit from below, ready to tighten the locking collar and one above, pulling the unit upwards and side-to side, maneuvering it into just the right position to get the rubber seal exactly centered.
(The sissor jack from your car can also serve to hold the unit up from below, while you fiddle with it from above.)
It is also possible that over-tightening the locking collar may distort the rubber flange, no matter what you do - not tested.
Given that there is no water pressure present, it should not be necessary to over-tighten the locking collar - it should probably only be enough to ensure that the unit is stable and sealed.
It's not a great stopper design and does seem to be heriditary, since the previous Kenmore/Insikerator unit I replaced also had a leaky stopper.
