Isn't the 112mbx a sealed cab? If you don't mind running the risk of doing it all over again you might try a sealed cab first. I think all those small Gallien and Kruegers are sealed, and a lot of DB'ers like that sound. A sealed cab is less efficient in the low frequencies, but the cut off of those frequencies is less abrupt. This works quite welll for most traditional amplified DB tones. Also, you need fewer calculations for a sealed cab, it's a pretty straightforward trade off: smaller is less bass, but will handle more power before x-maxing, bigger is other way around. So: build a cab at the maximum size you're willing to carry around and see if you like it.
If you want it ported you should calculate the ports on the exact specifications of your 12" speaker and on the frequnecy response you want. Ideally you'd want it to correspond with the gb 3.0 cab, to get the most out of the combination, but it will be difficult to guestimate a good match. Wether the ports are triangular, round, or on the backside doesn't really matter. It might be practical to use a standard tuneable port from the shop. This allows you to adapt the port afterwards.
