adding speed/vario switch into ASW-19 control stick grip

I installed a bycicle grip with a CNC end cap. The lifting end cap allow fairly easy installation of the ptt button and vario switch:
"Ragley Skinny Lock On Grip"

An alternative is reusing a ski pole grips. Those are the ones usually installed by the manufacturer. Leki has some nice ones and in ski resorts you can get them free because there are plenty of broken ski poles at every rental place. Installing button and switch is often not easy because you need to do it through the hole at the back for the latch. Get the smallest button and switch you can find!

Schempp-Hirth sells a foam grip with lifting cap, as installed in new SH sailplanes. Probably the easiest solution, but not the cheapest and I'm not a friend of foam grips.

Incase your stick diameter is too small you can use heat shrink tubing with glue to widen the diameter a bit.

I would definitely recommend a switch versus automatic switching. You need a good vario reading before the thermal kicks in and an automatic switch is usually to slow especially since most speed switches have a short delay even if the set speed has been reached.