So, your goal for hunting is a shorter rifle that is as accurate as possible; even though it will be loud? I would imagine that any air stripper that has a mating thread to the DFL adapter would do. Else, you can have one custom made.
The first question is what male thread does the adaptor have? I would imagine something like M18 x 1 mm, but without knowing that, you are hoping someone has the exact configuration you have in mind. Probably the reason for a lack of comments on your thread.
Are you sure that there is a grouping problem with the DFL moderator off? Perhaps grouping is adequate without the moderator and better with it on? With such a large caliber the projectile weight is likely going to be enough that air stripping may be less of a factor than barrel harmonics. What you might be looking for is an airstripper barrel harmonic tuner combination. But that will be longer and heavier, if all you believe you need is an air stripper. I mention barrel harmonics in case adding a short light air stripper does not produce the benefit you expect, and then you feel cheated.
You should have some idea of how long an air stripper you have in mind. The simplest kind would look a bit like a muzzle brake on a WW2 howitzer. Short and to the point.
If you take some pictures of your air rifle muzzle with the moderator off, and the dimensions of the handguard, I can cook up a design for a basic air stripper. The thread designation is the most important thing. Ideally, a simple brake or air stripper could be machined from aluminum. It could be made more fancy with a cone havening an adjustable stand-off distance from the muzzle.
Are you concerned with recoil reduction? If so, is that an important factor? I ask because simple perpendicular vents are the simplest to machine. Else, rearward jetting vents could be to reduce the recoil further, but it increases the complexity of the part.
Below are some ideas for a 3000 FPE 20 mm air rifle. The owner has a metal muzzle brake and claimed 3D printing was the only way to produce it. So, I came up with machined baffles and plates that could be welded together, if one wanted to. There are also some other brake ideas I did at the time.
The simplest brake is only show in section with two red arrows, indicating air flow direction. In one direction (A) the brake works best at air stripping. In the other (B) is works best at recoil reduction. The angle of the ports could be optimized for air stripping, but for the best combination, turned inserts can be added - as used with the other designs.
I see these designs as way overkill for your .457, as the recoil on that is nothing compared to the 20 mm monster. I have some other muzzle brake designs, if I can remember who I did them for, I can find the designs
