The best setup I have found without the use of flames is a combination of C-9 Christmas lights (the large old fashioned ones) and agrifabric tenting. I recorded a 5-6 degree increase from the outside temperature during a freeze two years ago.
Trees over 8' or so can be heated with just the lights to at least save the trunk and structural branches in the event of a freeze.
I'll also be curious to see what others use to protect their trees...

I am going with a portable greenhouse to put all my tropicals in except for my jakfruit which I am going to try covering with a tarp with christmas lights on the tree. I am also going to mound up the soil to above the graft line just so in the worst case scenario it will regrow from above the graft.

i have a barn to move plants into when it freezes.
I use the 1000w agrosun grow light to keep things
warm and the plants like the imitation sunlight
on cold winter nights. if it really gets cold
I use additional 1500w portable electric heaters.

takes a lot of electricity to keep things warm but
whats a hobby for if it dont cost lots..... lol

Thanks for the replies. I am considering putting in a Carambola tree in ground. It would be right next to the house, on the south side. I would keep it trimmed to 6 feet and cover it every night that freezing temperatures are forcasted.

Also, the idea occurred to me to either light one of the burners on the grill and turn it all the way low and place it near the tree on super cold nights, or build a small fire in one of those fire pit tables and leave the coals in it near the tree. Any ideas on how this could work?