Couple of things.... So far on the paper shim I haven't had any rust- that was obviously a concern however I suppose I am a "fair weather" hunter with a muzzle loader. I do finish a brown barrel with linseed oil so maybe that sort of acted as a water shield- although I've been told linseed oil isn't really that water proof.
On the dimple on the barrel's top- the four corners of the dovetail. That's how I was told so that's what I did. You need pretty good skill to do it correctly, the tapped down areas on the four corners can mismatch and not look very good.
Of all the various options I think a shim or a dimpling on the sight's bottom is probably the safest option for those with average skills. Aluminum foil is very thin- I have not used it but it may make a good shim.
One other idea- just thinking here...a drop or two of solder in the bottom of the dovetail and then tap in the sight- the lead is probably soft enough to deform as the sight is driven in.