Not a problem! Buy some black Tolex, cut a new complete back for it out of 3/4" birch plywood, install Tolex on the wood and enclose the back. Voila! Semi-instant bass cab.
Whether it would be desirable to add an internal baffle to somewhat separate the two speakers acoustically is another question. In my 60's Bassman 2x15 cab (similar design) there was such a baffle - really just a piece of 3/4" plywood that was mounted horizontally (when the cab was standing) equidistant between the two speakers. It didn't seal off each half, it just came within an inch or so of the back and an inch or two of the cabinet sides. I have some pix but they're in the computer at home and not available online...yet. Send me a reminder and I'll post them.
Others may have advice to share about whether it is desirable to partly or fully isolate the speakers from each other in the cab.
After closing the back, you can do some figgerin' with WinISD Pro (see
http://www.linearteam.dk/default.aspx?pageid=winisdpro) and check the performance of various speakers. Good luck with that program - many people use it but I keep being unable to understand it.
Note: if you're going to use this cab with the Coliseum-300, this is a GOLDEN opportunity to find speakers that you can configure to make this a 2-ohm cab!!! That will open up all the power of the 300 to you! Or - if you'll be using it with a separate cab, I'd configure for 4 ohms and have a more versatile cab that can be connected with just about any other cab you might own in the future...as long as you're using the Coliseum-300 with its 2-ohm tolerance.
One more thought...if you go for the 4-ohm configuration, you could buy a pair of 8-ohm speakers. That would give you the option of wiring the cab for 4 ohms for use with the Sunn, or changing the wiring to 8 ohms if needed.