Yes, you can get another piece of rubber. I mounted some handguards a couple of weeks ago and ran into a similar situation. The tube needs to be big enough inside diameter to go over the metal piece while still being able to go inside the bar. I would suggest trying the supplied rubber again, but this time maybe heat up the rubber with a hair dryer first and grease up the metal so it will slide on easier. More than likely, it will go. If you get it started, tap it a bit with a mallet, or 'screw' it on with channel locks.
Another tip: You may need to bend the guards a bit if they don't match up with the bar mounting. That is a tedious proccess, but may be neccessary.
Once you get the rubber thing worked out, just push the guard into the handlebar and sort of "eyeball" it and see what bends need to be made so you can bolt the other end on. I think the optimum position for guards is the front of them a little lower than level. I'm not familiar with the Moose guards, so this tip may not apply to you.