I have used heated insoles in the UK – probably a different make –
supplied from a 12v battery. I needed to switch them on and off after a
brief period until really high and cold, as they got quite hot. Even at
very cold ambient I found that I only needed about a minute or two on at a
time, then off for longer.

I wear surgical boots, not easily replaced with commercial insulated
footwear, so for extra insulation I made some over-boots out of false fur,
which lace up. I put them on just before getting into the glider. It can be
a bit too warm low down, but I usually soon get to an altitude where they
are fine.

I bought some heated socks once, but they only heated the toe area – I
have bad circulation in one foot and need the whole foot to be warmed. The
insoles do that.

Chris N.