Hey Robert-

I wouldn't worry about it at all. I don't think you'll need to shim it. When you stretch the skin to the coaming it will actually flex and may even touch the isserfik depending on how tight you are able to get it. If not- keep in mind that the coaming doesn't need to support any weight and it's not structural, It's held in place by the skin regardless of whether or not it rests on the isserfik. As long as it secures your tuilik/aquilisaq/spray skirt you're fine. A lot of builders of rolling-specific boats will purposely move the isserfik back behind the back edge of the coaming so that the two have no contact at all. That allows the coaming to flex out of the way lower when doing layback rolls so that it's easier to lay flat on the aft deck without being overly flexible.

m