I use wood shims at times for a quick fix if my accessory isn't drilled for my "wobble bar", descibed here: A better way to do it is to use a 1/4" steel bar about 2 1/2" wide and maybe 6" or 8" long, drilled and tapped for a about a 1/2" bolt about 4/5 of the way from one end. Bend the bar about 15* on the opposite end from where you plan to drill it at about the 1/4 of length mark. Use a bench vice, and a BFH, maybe some heat too.
Drill the extender using a 5/8" bit, big enough to accept the 1/2" bolt. Put the bar on the bottom of the extender, bent end underlaying the tail end of the receiver and tighten the bolt. Tightening the bolt will force the end of the bar near the bend to "cinch down" on the receiver, the drilled end to "cinch down" on the extender or accessory, eliminating wobble.
In addition to other tools, I keep a dedicated box end wrench in the jeep just for tightening the bolt. A bolt and nylock nut would work too, I think, if you don't have a cheap/easy way to tap the bar (but you would need two wrenches). One bar works for many accessory racks or extenders, so long as they are drilled to accept the bolt.
BTW, the idea isn't mine, a fishing rod rack/cooler rack that I bought came with one. Works great, eliminates the wobble, which I find really annoying, especially if whatever accessory rack I'm using is in the front receiver.
Hope this helps,