Product review (1 stars): Beware if you own an older KitchenAid 18 model
You might have ordered the KitchenAid 4318938 and received the W10165296RP as the substitute since KitchenAid has discontinued the 4318938.  It is extremely difficult to get them to fit since they are a bit smaller.  You really need to work to stretch it to fit.  The other disappointment is the button holes in the bag do not align with the buttons on the compactor, so forget about buttoning it in and concentrate all your efforts on getting it to fit. Oh and they're thinner too.

As another reviewer noted, the Whirlpool W10165296RP IS NOT a substitute for the Kitchenaid 18" 4318938 bag.

Someone told me the Viking bags may fit and they're supposedly thicker.  I'll try those after I go through the 45 W10165296RP's that I am now stuck with, if I have any fingers left.