I've found that about 80-90% of ferals will turn friendly if given the chance. Some more so than others, though.
Of the last batch that we worked with, only two kittens were completely un-catchable and were left in the colony after TNR.
The other 18 got adopted, plus the 3 that we kept, and half a dozen adults the residents kept.
We're building something similar in our 13x40 screen porch.
Not gonna use a lot of consumer grade fabric, though as FL humidity is very unkind to that and it molds and/or deteriorates quickly.
I'd be leery of a heater, or really any electrical cord left unattended.