How bad are your frosts? And how many hours? I have an orange tree
and I've been able to sneak through a light frost for a few hours.
Here in California they use little gas heaters for the citrus orchards.
Maybe some kind of poultry heater would work too. If you don't have
one handy you can improvise. I fill a clay pot about 3/4 full with
sand (coarse or fine). Then I light a few pieces of charcoal. By a few
I mean 5 or 6 pieces, small enough to fit in the palm of your hand.
Feel free to roast marshmallows while you're waiting for them to die
down. After the coals die down I cover the pot (full of coals) with
another clay pot that has a hole in the top. I put the pot a few feet
away from the trunk.

You don't want a lot of heat. You just need to keep it slightly warmer
than freezing as long as possible. And be very paranoid about using
open flames outdoors. Be sure your pot won't be disturbed and can't be
knocked over. And give yourself a few feet of clearance around the pot.
Be sure it is covered and secure before you walk away. My tree is in
the middle of my lawn and the nearest structure is 15 feet away. Don't
do this if there is a breeze.

I've thought about filling a small barbecue or cast iron hibachi with a
concoction of stones and sand to see if it says warm longer. I suppose
if I had to do this more than once or twice a year I'd invest in a real
heater! Anyway, that's my crazy idea for you. Good luck!