I did this too, when I replaced both axles last month. Then I drove down the highway from Alaska to the lower 48. No problems.
I used the "collar" part of the old worn out boot, which I cut from the old axle, and slipped on the new axle. Then I used a nifty Clamptite tool, which allows me to make a helluva good clamp with minimal fuss using wire. Clean, clean, clean.
Now, how to get the new grease-filled boot over the top of the obstacle I just clamped on the axle. A spoon. Yep. A normal kitchen spoon. I inserted the spoon underneath the NEW boot (where all the grease is), turned it a few degrees to engage the shape of the spoon, and with the help of a pair of piers on the handle of the spoon, pulled, cussed, pulled some more, cussed some more and slowly, carefully, while pulling the spoon, worked the new boot over the top of the old collar. Presto! got it! Then I used my Clamptite tool again to fashion an all-too-slick wire clamp over the top of the new boot. No leaking, no slipping.
If you've not heard of a clamptite tool, you're going to freak when you see it. It does ONE THING really, really well.