30dayreviews, w/ the skeletal joint ahead of your door i might suggest placing your seam on the aft edge of the door opening which would provide the least possibility of issues
that would automatically provide a reasonably good butt joint backing butt block

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on the outside i'd suggest using H. H. "Dynamite" Payson's method to hide the seam AND reinforce it

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when hollowing the underlayment out try knot to go thru the entire thickness(2/3)
once a layer of cloth is in place make sure it is totally saturated
hand block sand to surface level
BON CHANCE
sw