UStik

I'd say you're on the right track: tow-release lower than C/G and wheel axle below wing's leading edge (old rule of thumb).

And you'd need an unusually fast towplane (due to high wing loading of the Cub). After hand launch the plane seems to be just lowering it's nose to gain speed (means enough pitch stability) but there's simply not enough height (potential energy) to get it and establish on a stable glide slope.

Thanks for this thread!