Go to Walmart and buy a small packet of flat washers, preferably as close in ouside diameter, to the inside diameter of the receiver tube sides you want to shim. Gently (or not?) tap them into the receiver with a screwdriver and hammer, or similar, until they fill the gaps and shim up the hitch/shank. Helps if you (or a helper) can wiggle the shank a bit while you're inserting the flat washers. Then use a couple of cable/zip ties placed close to the receiver opening around the shank to keep them from escaping.
Total cost, less than $5.