Turn it into a medium mount Vulcan, frag damage and all. No, really, I'm not kidding!
Currently the HBL is operating under the same design restrictions as the small energy PD: because high tech energy pd using ships are (usually) mobile and have good shields, their PD is worse at stopping area saturation missiles, which with a single exception are unguided. Instead, they are better at kiting, with long ranged precision, which is what high tech ships try to do. How a wolf with beam pd deals with a mess of harpoons vs a lasher with vulcans is a good example of the 2 doctrines.
A lot of ships have lots small energies, more than small ballistics. Not many high tech, high mobility ships have spare medium energies though - they are needed for offense. I actually can't think of any except maybe a 1x heavy blaster Medusa (which is arguably just not good). But there are a good number of lower mobility ships that have spare medium energies. Apogee, Eagle (moderately mobile, so maybe doesn't belong), Paragon, Pegasus/Executor, Champion (Ok they are hybrid but still), probably a few more. They could all use some strong anti-area PD options. Arguably 2 of these (Apogee and Paragon) have systems to help deal with this.
Medium ballistic already has flak for "aoe goes boom" point defense in this size category and deserves to keep that unique, so give energy a big old spray of volatile plasma or pulse laser minibolts or... etc. Its not going to make high mobility ships have too good PD because they don't have the slot type to spare, but it would be an interesting option on all the slower or more flux starved ships.