Yes, polyfill's fine. When I had a pair of test mules, other than the main chamber (like many scoops it pulls double duty), a bit around the top bend was all I found necessary on my low output impedance amplifier to kill unwanted midband output, which is what the damping in a back horn is for.
Since that's not an option for you, or at least somewhat difficult, I'd probably be inclined, if you've got excessive midband output through the horn (there shouldn't be too much present though), to put a nice pad of whatever you have to hand on the internal back-wall, above the deflector in the terminus (mouth). That should do the job reasonably well, & you should be able to keep it out of sight.
Since that's not an option for you, or at least somewhat difficult, I'd probably be inclined, if you've got excessive midband output through the horn (there shouldn't be too much present though), to put a nice pad of whatever you have to hand on the internal back-wall, above the deflector in the terminus (mouth). That should do the job reasonably well, & you should be able to keep it out of sight.
IME Polyfill just needs too many layers to be effective as lining. Much better to go with mineral wool, if you don't mind working with it, or thick (ca. 2 cm) cotton felt that is not to dense.
I remember going from just loosely stuffed rolls and pads of polyfill to cotton felt on my first 126 speaker. The difference was dramatic. From barely being able to understand more distant voices and having a faint box sound, to voices being among the best I've ever heard on a speaker and absolutely no box effect. More bass to boot, because I could do with less polyfill for stuffing.
I remember going from just loosely stuffed rolls and pads of polyfill to cotton felt on my first 126 speaker. The difference was dramatic. From barely being able to understand more distant voices and having a faint box sound, to voices being among the best I've ever heard on a speaker and absolutely no box effect. More bass to boot, because I could do with less polyfill for stuffing.
No doubt, but since I designed the BK-12m, I believe that I might have an idea as to whether polyfill / Acousta-stuff does the job required of it or not. 
It does.
With that said, note that there is polyfill & polyfill. It's not all created equal. Acousta-stuff, which is the specified material, is at the upper end of the performance for this type damping. Some of the cheap solid strand types conversely aren't anything like as effective. Fiberglass can be better still, but again, depends on the type used.
It does. With that said, note that there is polyfill & polyfill. It's not all created equal. Acousta-stuff, which is the specified material, is at the upper end of the performance for this type damping. Some of the cheap solid strand types conversely aren't anything like as effective. Fiberglass can be better still, but again, depends on the type used.
For those interested, while I read several reviews of the FiiO, this one helped me pull the trigger.
Fiio A1 Digital Speaker Amplifier – Full Audio Conquest? | Headfonics
The amp showed up. It was packed really nice. I really like the case. Very light. Just over 1 lb. IMO, the construction of the case is really nice too. I'll be testing it out later, hopefully this weekend. Long days at work have this old man a bit tired.
Fiio A1 Digital Speaker Amplifier – Full Audio Conquest? | Headfonics
The amp showed up. It was packed really nice. I really like the case. Very light. Just over 1 lb. IMO, the construction of the case is really nice too. I'll be testing it out later, hopefully this weekend. Long days at work have this old man a bit tired.
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