At that size, I'm wondering if the tent is really suitable for touring. My two person tent isn't much over 5 inches in diameter, and a touch under 18 inches long, and it was made in the 1970's. Unless you're carrying the tent for a larger crowd, that sounds like overkill.

You're money might be better spent on a better tent that will fit on a conventional rack, rather than building a rack to fit your tent.