If you make your own there's a couple of things I would suggest.

First off make the cradle high enough that the glider's wing is spaced about one glider wing chord or more above the carrier plane's wing to help reduce the interference between the two wings. This is just common biplane design knowledge and you ARE making a biplane even if it's with two models.

Second I would set the angle of the upper cradle wing saddel so the glider sits with about 2 degrees less incidence than the carrier plane's wing. This will ensure the glider wing won't tend to stall and cause a high drag moment during the climb which could make the carrier try to pitch up. This angle will mean that the glider's stab is adding some up trim but a bit if down trim on the carrier model's elevator should cure that.

Lastly, the larger the carrier the less effect the glider will have. So if you can talk someone with a 100 inch model into adding the glider carrier it would be best. But as you can see there's folks with Kadet 40's and stuff that have done it as well.