I lost an m7 set screw. What do I do?

You’re in way over your head. Get a professional installer so you don’t end up with an electrical fire 🔥. 

If the space around the bolt is fine, then the bolt should be good until you find a proper M7 set screw.

The advice is to make the bolt the ground lug one, meaning if the missing set screw is on the power or speakers, move the set screw of same size from the ground over to the other terminal, and then use the M7 bolt for the ground wire tightening.

That way, if the ground 'bolt' is accidentally grounded to the chassis of the car or the amp, or whatnot, it does not matter...no current will flow as it is already at ground potential.

Again, if they are all m7 connections, make the ground connection the one with the bolt. You'll be fine that way until you get to finding a proper M7 set screw.

My local Ace Hardware has metric set screws.

Thanks for the advice! I ordered a good kit for this amp. The only thing missing was that one set screw. When I wire it this week, ill use the bold (unless I can find a new m7 set screw) to the ground terminal.

Its a jbl BP1200.1 mono amp (for car audio). I feel confident in my skills here, just wanted to get some extra input.
Thanks

Worst case scenario, would be to cut the bolt you have to the proper length and cut a slot in one end that would allow a screwdriver tip to fit. Do you have a Dremel tool? That would make life easier.