TFG
You don't have to have any cover outside controlled airspace but it would be prudent to try and obtain a service from someone even if only F.I.S.
It IS a big sky yes, but the thought of someone else coming your way in that nice fluffy cloud is quite scary!
Personally (and professionally) I always upgrade to R.I.S whenever the need to fly I.M.C arises.
Happy Landings
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JWF