What ohms does your amp require? I really doubt you'd get an ideal impedance for any guitar amp as most guitar amp speakers are 4, 8, or 16 ohms and most amps are made accordingly. As far as The cab, it's a box wrapped in tolex with a cloth grill in front of a baffle, it's easy as pie.
Quote by R45VT
depends upon what you are expecting to get out of it. Is this for guitar? Are you using a couple of cheap speakers just for a fun project or as a stop gap until you can afford to upgrade?

If that is the case then just build them into a fairly compact box, something like 50x25x15cm make it airtight and away you go.

If you are looking for something high tech then there is a lot to designing a cab and I'm happy to help