My bushbuddy fits pretty good in the snowpeak 900, probably a 1-2 mm clearance all around. It does not really rattle around, but there is some space. When I was using that pot I generally had the bushbuddy in a small plastic bag and inserted that inside the pot, and it was a nice snug fit. This was really unecessary, and I don't do it anymore.
The firelite pots are just slightly larger, maybe a couple of mm. The bushbuddy fits good inside, but it's not a snug fit.
How much clearance is there between your bushbuddy and the walls of your SP900? Sounds like a lot.