No back chamber is needed on this horn.

28cm^2 throat, 3250cm^2 mouth, 43cm axial length. Have the driver mounted right up against the mounting plate, and the filler block there to take up ~16-18cm^2 to smooth the top end ~1.1db or so.

Heres a filler block that I made before on a tractrix horn. Once the driver is mounted the filler block will be in front of the cone as you can see. Make sure that you leave enough clearance so that the cone doesn't hit the filler block, thats why its important to take precise measurements of the geometry of the cone, spider and dustcap so you can do this while also having it take up the right amount of room.

Adrian