Year Really! 
It is very important problem, for me too, I create Flows for processing big files (~20-30GB) and prefect use tmp dir for store it (I noticed it when Flows was running), prefect cleans after flow executed it is good but I’d like to set the dir for temp files cause boot memory is 50GB only and the boot memory is expensive to use and the better way is to use extra-disk or may be store every data at .prefect/ (which location we already can control)
Now I see just mount extra space to tmp dir
Thank you 