It is quite easy to construct a list of 26 seven-letter words in which the middle letter takes on each alphabetic value. To make this word game more difficult, I have added the requirement that the first three letters and the last three letters must both be words. No doubt the hardest middle letter to deal with is Q, for there are few three-letter words beginning with U and a vowel.
Meyer, John M.
"A Positional Word List,"
Word Ways: Vol. 11
, Article 19.
Available at: http://digitalcommons.butler.edu/wordways/vol11/iss2/19