Read the  "Scaling the HWK for 18" thread. (2 down from this one right now)
General discussion about scaling.

I have researched the old archives and all the ones discussing
scaling are no longer available. Also I haven't had any direct response to the question myself.

If you have the specs for your driver
use WinISD to figure out a 4th order bandpass. Use the rear chamber measurements (cubic feet / liters)
and adjust the HK plans so it's center chamber has the same volume. Look at the dimensions and subtract from the original then take the difference and subtract from the top/bottom sections.

If you don't have specs.
PM me as I am pretty sure I can't post the measurements here (?).