Who makes the heaviest gauge, tightest fitting receiver covers for milled or stamped receivers?

tight fit has little to do with the cover and much to do with the builder of the weapon.

OK, aside from shoddy builders, same question.  Which type has the potential to be the best fitting?

any of them would have the ability to fit well, you just need to bend out the rear portion a bit to tighten it up if it is very loose.  

this is a bit of an odd question, are you looking at mounting optics to this cover or something?

Yes, I need the steadiest cover I can rig.

get a side rail and mount optics to that.  

Buy a Galil cover, it's the only one that has any chance of working.

AK top covers are flimsy pieces of shit.

Quoted: Buy a Galil cover, it's the only one that has any chance of working. AK top covers are flimsy pieces of shit.

If you get a Galil cover you'll need the spring assembly as well. The rear tang on the back of the AK spring guide usually isn't long enough(in my experiments) and the Galil cover will sometimes pop off during rapid fire ...
Unless you do something like Krebs has done where the scope rail locks into the back of the rear sight block to prevent movement in the front of the cover it's not going to maintain zero and then it's "still" going to move. I just got through with two months of experimenting with this, ended up ruining a perfectly good, impossible to find Intrac rear sight block and said "screw it. So I just converted the rifle to a 104 look-a-like and it works fine ...

The bottom line is that topcovers were never meant to have an optics mount ...