I have a used upper. The front sight assembly was attached by regular roll pins that were resting against grooves on the bottom side of the barrel. The sight was canted so I drifted these out just to see how tight the assembly was, and how easily I could rotate it. The pins came out easily and the whole assembly slid of the rifle easily. If I just put it back together the roll pins will sit in the grooves crooked and recreate the cant.

How can I attach this base securely and then lock it in place without a cant?

can I use adhesive of some sort on the sight base before I even pin it?

Can I tap a hole on each side of the sight base and put some set screws in?

Any suggestions would be appreciated.

bacon taco taco

Yes you can tap the holes to use set screws, but you may want to have the barrel checked out by a smith first to see what the real cause of the problem is.  The FSB is supposed to be held on by taper pins, nor roll pins, so the quality of the barrel might be questionable.

Think about it.If it should have or originally had taper pins the roll pins would cant the sight.Look to see if the pin grooves are tapered to one side.