For this cover, I used the upper image of Doctor Strange from the cover of Strange Tales #125 and added that to the interior art splash from this issue.
INSIDE FRONT COVER
Inside front cover: The cover to "Nightmare", The Doctor Strange Marvel Novel Series #7 written by William Rotsler, published by Pocket books. Illustration by Bob Larkin.
INSIDE BACK COVER
Inside back cover: A collage of Jack Kirby drawings of Doctor Strange. The central image is from a 2 page magazine splash for an article about Marvel Comics in Esquire Magazine (Jan. 1966) (It was reprinted in the book "The Silver Age of Comic Book Art" by Arlen Schumer.) The background is from an issue of Kirby's 2001: A Space Odyssey, and the other illustrations of Dr. Strange are from Fantastic Four #27 and Journey Into Mystery #108.
I always use the original back cover on the customs.