I can't speak to your question, but a suggestion along those lines would be to ferment in a corny keg. a 3gal batch is a great, easy size to ferment in a 5gal corny and they are way cheaper than the brewtech stuff. I do 5 gal pressure fermentations in a corny, but it will be fine without the pressure too if you don't want to go all in down that path.
I do my cider in 3 gallon batches and keg it in a 5 gallon keg, no issues with the head space as long as you are purging it (which you can do for free if you are fermenting in a separate corny)