I suggest a second sticker on the vario.

My first sticker says “gear”. I stick it on the vario when launching.
If it still there when I come off tow and start thermalling etc., it
impedes my view of the vario and reminds me to raise the
undercarriage. When I have done the latter, I move the sticker to the
airbrake/spoiler handle. If it is still there when I approach for
landing, I have forgotten to lower the gear. When I do lower the gear,
I move the sticker back to the vario as I don’t need it for approach
and landing.

For the USA transponder/PowerFlarm, I would have a second sticker on
the vario saying transponder/PowerFlarm. If still there when looking
for lift, it would mean I forgot to switch them on after pulling off
tow.

Cheap, fairly foolproof, and the only time it let me down (or rather I
goofed) was when I forgot to put the sticker on the vario in the first
place, and ended up landing without lowering the gear.

Chris N.