Most clip on thermals and NV devices are set for 1.5" (though a few are 1.35").
The easy answer is to just purchase a unimount with a 1.5" height.
I would not shim a single ring. While some may say it works, you would be essentially bending the scope tube, which is not a good thing. The issue isn't so much damaging the tube, rather you could potentially bind the erector assembly (the mechanicals that hold the lens in place internally), especially if this is a front focal plane scope. If it binds, then the erector will not track correctly when making adjustments, and may even damage the erector assembly under recoil.
If this is a second focal plane scope, then you may get away with shimming the rear ring (a SFP erector assembly is not nearly as complex or big as a FFP erector assembly), but it's still not a good idea.
Think about this way; How much did your rifle cost? Is it really worth skimping on the right mount?
Remember; Your scope is the weakest link on a weapon system or hunting rifle. Act accordingly, and ensure you have the most robust mount and optic you can justify for the rifle's given purpose.
BLUF: Don't be penny wise, and pound foolish. Go get the right scope mount.
I personally prefer these, or a Spuhr, but you need to decide how much you are able to spend to correct this problem.
M-BRACE SCOPE MOUNT
JMTCW....
