How to build this?

>
> > Between each layer of seals use vacuum pumps to minimise any
> > leakage to space. Vacuum pumps are capable of reducing the
> > remaining leakage down to countable molecules if you really want
> > that.
>
> Do you think that this type of arrangement inserts a critical point
> of failure if the vacuum pumps and their back-ups fail through some
> type of localized catastrophe.

Well, in a catastrophe kind of scenario you just close pressure doors
either side.

Otherwise; pump failure isn't at all a catastrophe. As I noted, the
natural rate of leakage, provided you use reasonable seals, is really
rather low; perhaps a few cubic feet of air per hour. The gap only needs
to be a few thousandth of an inch.

The pumps are just there to scavenge the atmosphere you would otherwise
have lost to space; atmosphere is expensive.