Adjusting McKesh type mirrors

I said "McKesh type" because I just got the Eagle Vision mirrors from Camping World. They look to be knock offs of McKesh.

In reading numerous posts about mirrors, I remember seeing a note from someone about using a small wood spacer between the door and the window glass to keep the mirror clamp from depressing the rubber seal that is against the window. This seal is actually designed as a "de-mister" for the window and not necessarily to keep water out of the door. None the less, preventing a permanent depression on this seal would be desirable.
Has anyone tried this spacer and what were the results?
I'll be using these new mirrors this weekend so I can report back next week on how they work.
Thanks.
Alan