Coleus are not frost hardy.
I made a cello a while back if that's any help ;-))

Probably not. Coleus aren't hardy and the Scaredy Cat is usually treated as an annual, grown each year from seed. It'd be safer to bring them indoors for the winter.
Can't help with the violin maker - do you want to play the fiddle?? {another joke}

TA again .Hope to buy a small cost effective heater that won't damage the cover.AS for violins ,first catch your cat:

Buy a water-pistol, Shrimp! That helps.

Hi Shrimp! If the greenhouse is small enough, an electric space heater may raise the temperature just enough. Be sure to use an outdoor extension cord, or run electrical conduit to the greenhouse, use a ground fault circuit interrupter, and stand the heater on a flag in a dry spot, for safety. Paraffin and propane heaters can damage plants--and sometimes gardeners--with toxic fumes!