expansion tank replacements

[koabi.brooks at gmail.com (Koabi Brooks)]

A continuation of the bored at work series where I ask random things.

There are tons of overflow tank options, some that look pretty nice.  If I
used something like that but put in a "feed" line like how ours are, could
that be used as an expansion tank?  I assume I'd need something I could fit
a radiator cap onto to bleed off pressure in order for this to work.

I ask because i have a mkIII bottle that's zip tied in.  I can do better
than that :)  but i don't really want to weld in the bracket for the mkIII
bottle if I can put something nicer in.

still bored,
koabi