MH and DIY Cool-Tube "Bake-a-Round"

I have a question about using a 250MH in the Bake-a-round. The bulb just fits inside the glass tube with very little clearance, I'd say less then a MM. Do I need to be concerned about the expansion of the MH bulb cracking the tube? Or would they expand together?

Thanks.

After thinking about what I asked, it doesn't make any sense. If there isn't any clearance then there won't be any air flow.

Thanks anyway.