How do you get the screws tight enough in a metal fence?

Coat the screw threads with Lock-Tite before screwing them in. Lock-Tite can be purchased at any hardware, or auto parts store. This will do the trick, Good luck

You could try using felt, washers (or another dense, soft material) to make rings around the screws where they contact the fence, and tighten the screws with two bolts if one does not suffice.

Use special lock washers that keep the screws/nuts in place.

The best solution is to get steel lock washers and install one under every nut.