You really had me with this one for a second Mikling, you are of course referring to the orange clips for the bottom of these cartridges.
That’s a good idea, except for one small thing, what about the silicon washers, how do we make these silicon washers, because without them the whole project would be futile...
I imagine one can use a bicycle tube and cut it up or some kind of silicone rubber kitchen mat. It does not have to clip on perfectly. One might be able to hold it on using elastics. Many of these kind of carts have had the shoes thrown away and then the users realize they need these and then cannot find anywhere to get them.