Shim will work fine, but may want to go with a Accuwege isntead.



The accuwedge has some compression to it, so it a little easy to slip the rear take down pin when you push down hard on the upper to lower instead.

The down side, you have a high shelf receiver as shown on the left, so would have to trim the bottom of accuwedge to get it work in your rig (since the accuwede designed thicker top to bottom  for the low shelf receivers isntead).
.


As for your shim, it working pretty much the same as the receivers that have an upper adjustment screw that allow for the same as your shim is doing.



So if nothing else, if the shim you are using is not made out of plastic or nylon (has a touch of give in it) might want to change the shim out to a piece of nylon block instead.

As for up front, if you have wiggle there, easy to solve with a O ring instead.