The construction of forms (crossword puzzles in geometric shapes) was a popular pastime among puzzlers in the late nineteenth and early twentieth centuries; scores of examples are given in A Key to Puzzledom (New York: William W. Delane), published in 1906. More recently, the late Palmer Peterson of Lennox, South Dakota, exhibited a Hollow Diamond in the May 1971 Word Ways.
"A Hollow Cross,"
Word Ways: Vol. 12
, Article 15.
Available at: http://digitalcommons.butler.edu/wordways/vol12/iss4/15