Unfortunately, once the band is installed, it can't be tightened. Once the band is pulled tight, the collar is crimped down to hold the tension on the band.

It's not likely that you'll be able to get it any tighter yourself without that special tool. You can take it to an axle shop & get'r done right or run zip ties like LittleBeast & tom4bren (kinda puts you in a special crowd)