Try foot petals. They are not nearly as wide as traditional heel cushions. Apparently they aren't on Nordstrom.com by themselves, but you can see what they look like:
http://shop.nordstrom.com/S/3110436
Or, have you thought about trying a ball-of-foot cushion? This might help keep your foot from sliding forward and therefore fix the heel slippage issue?
I often have the same heel slippage issue and the foot petals work well; they don't take up too much room and if you place them along the top edge of the heel of the shoe they leave enough room for your heel while gripping your achilles tendon/ankle area, if that makes sense...
they don't solve EVERY heel sllippage shoe for me, but they do help if the shoe slips only a little.
