How to mount a sight to an 870

Grandpa wants to mount a Holosun to his 870 20ga turkey gun. I'm looking for the easiest/cheapest way to get a pic rail (or similar) on top. It doesn't have to sleek or pretty. Function over form so to speak.

Any suggestions from you guys? Gun isn't drilled and tapped and he said it would cost $100 to do that so he's not biting.

submitted by /u/nick91900
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