I have done this with the Hangar9 cradle. I didn't find it necessary to use wing supports, just made sure there was plenty of tension in the rubber bands. I think I used at least 3 per side. I did have the carrier break once, when one side released and the other didn't. The bands got cought up somehow, and one side of the cradle actually ripped off and stayed on the glider wing.

The cradle was mounted on a trainer60, and was used for up to a 100" glider. THe bigger the glider, the more it's influence on the tug. On full house gliders 2m and bigger, the glider can actually steer the combo around the sky. On the 100", I actually had more authority over the way it all flew than the tug pilot. We'd both pay carefull attention on takeoff to make sure there were no strange occurences due to trim differences. Once the combo was trimmed for stable climb, the tug pilot would let me steer using the glider and only help if things got interesting.

Do the release by cutting the throttle during climb and keep the nose of the combo up until quite slow. The higher wing load trainer will then drop away at release while the glider should happily fly away at slow speed.