The Blackstar HT-112 is the guitar speaker cabinet that matches with the Blackstar HT-5 head both tonally and in dimensions.
Here's what Blackstar say about the HT-112
The HT-112 extension cabinet contains 1x12" custom designed Blackbird 50 speaker. It has been produced to the highest standards and using the finest materials. Every care has been taken to ensure it produces the best possible sounds. It really will last a lifetime!