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It's a common problem with that (cheap) design. The top does indeed wobble if screwed up any distance, which affects line-of-sight to the crosshairs and therefore accuracy.
The problem with glue alone is that you don't know that the top part is straight/centred when the glue sets.
The top of the eyepiece unscrews completely, so I used cardboard shims (with a hole in the middle - trial and error to find the right thickness) so that the top part could be screwed down tight (and straight) at the right focal point. I later found some rubber mat that was the right thickness and used that as it was less likely to unscrew compared to cardboard. A large machine washer might also work.
If you try any of these and find they work, you could glue them in place permanently, but I don't see the point as you can simply tighten the top part before use.
HTH
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