Look on page 373 of Norman Freidman's "U.S. Small Combatants" for line drawings of the mini-ATC.
Note that there is a picture on the bottom of page 374 that is captioned as a mini-ATC, however it is actually a Medium Seal Support Craft (MSSC). They look alike at first glance but there are major differences.
You can get a copy of the book through your local library's Inter-Library Loan program if you don't want to purchase one.
Hope this helps,
Matt