Adhesive to use to help screen stay on tight?

Here is the thing, and this should be notated with all customers you deal with.

If you open any unit, you are risking it's water resistance ability. Samsung does not supply their tools to accurately seal devices.

This will also depend on the unit you're working on, I'm going to assume it's a glass back from what it sounds like. If you apply too much heat or use slight force to remove the back, it will be slightly bent out of shape, which is very common. Think of it like tempered glass. Once you apply it and try to remove it, you will have some lift. If this is the case you want to order a new back glass and new adhesive piece for this repair to get the best result.

We attempt to add slight weight when we put the unit back and align it to the best of our ability.

Even with that done, you are still at risk of losing water resistance. The other option, which is believe is more suitable, it to purchase a case.

Hope this helps.