I understand your arguments for this choice. Copper and a thin pad, but I think it wil not be practical.
First the price of the Panasonic is extreme high in comparision with other thermal pads.
Second I think copper and aluminium is not a good combination because of the electrode potential of metals, one of the metals will be corrode. In this case the aluminium casing. Corrosion is an isolator, so the effect you want to achieve is lesser you want. If you use aluminium for that ring (conductivty of 250 W/m-K) then there will be no problem.
Last point is, you want contact with the casing as much as possible. Then you need some elasticity from the thermal pads, wich is less with thinner pads.
Imho, a better choice would be a half ring of aluminium, what has a snug fit inside the front of the casing, sticked to the laminated metal of the motor with elastic thermal pad and also stick inside the flat side of the cover casing an elastic thermal pad ring that rest a the top of the motor after fastening the cover.
Why a half ring. This because there must be some pressure to it if you mount the cover casing.
I think this is almost impossible with a sticked full ring.