If you want to keep the price down, and you are not worried about
certification, you could use a home oven temperature probe. There are meat
temperature probes from cooking equipment suppliers and actual oven
temperature probes available through pottery places for kilns.

Some of these are selfcontained and the probe could be placed in a sensitive
area and the cable fed through to the cockpit or cowl mounted gauge. The
cowl guage could be read via the windscreen, or a mirror.

Some of these can be set to a certain temperature to turn off the oven when
the meat is cooked. This could be used to operate an "Idiot Light".

Hope this helps,
Peter