I have been putting fermenters into a fridge while they are @ mid to high 20*C by using a portion of boiling water to make up the volume.
I find the fermentation itself keeps them warm to drop SG most of the way.
I do have a 1000w floor heater installed in one fridge that's wired up to a STC-1000 dialled in @ 26.5*C
Runs for 10-15 secs very rarely.
I think the trick is to get the volume up to where you want fermentation to take place when the yeast is pitched and between fridge insulation and the fermenters thermal mass, the fermentation should hold a reasonable temp
I do 46L batches in 60L fermenters which take up a significant chunk of the fridges internal space
Cheers