Firstly, don't sand it.......it's not the way to go on Carbon parts unless absolutely necessary.
There is quite a difference between various bottom collars and steerer sizes, it can be a bit of a problem.
You'd be better off taking a Dremmel to the bottom crown ring rather than the fork.
Attention needs to be paid to make sure the crown isn't left with burrs or edges and needs to be a nice fit.
A gap isn't a good idea....it'll lead to an area where the stress levels are raised and could lead to failiure in the longer term.
If neither of these are possible a good solution could be to add a spacer under the crown that'll fill the gap (but that is a final solution really if nothing else works)
Years ago when full carbon forks first arrived on the scene I had a similar problem with a LOOK fork, I ended up filing it down to get a good interference fit. I was working in the Composite and Laminate industry at the time and came to the conclusion that there was enough material around the crown and steerer to make it an acceptable solution. But these days some of the forks are very light in material weight around the steerer area (in the search for weight reduction)
What I'm trying to say is......only mess with the steerer as a last solution.
(edit) If you want to send me some pics or a PM, to assess the state of the fork/crown joint feel free.