A detolf is great! Lots of room but it might take him some time to adjust if he's been in a little tank or breeding bin.
Hamsters, as you know from what you said, are naturally wary of things coming down from above them. To help him feel safe consider covering part of the top with something darkish-- making sure he has adequate ventilation. Some people cover a corner too (on the outside) to help hammie have a place they feel safer.
Let us know how you get on!
