Actually, there's a better way to do it.

What I did was to build a cradle that mounts to the top of the wing on the tow plane.

The glider sits in the cradle and is held in place by rubber bands.

When you're ready, the rubber bands are released and away goes the glider.

Two big advantages to this system are:

1) you don't have to worry about what position the glider is in (Too High, Too Low, etc)

2) you can use ANY glider - IT doesn't need any type of release mechanism