Looks like about 3-5L IMO. The main thing is to keep the front baffle about the same size. What is that? A 4" kevlar driver? Front Baffle looks about 6". Won't be much affected by the bass performance since it's not doing much down low, so the enclosure size should just be reasonable.
Ialways thought those heads were the cat's Pajamas. Maybe i shoild make a fake matrix like head for the LM-1s to add to their appeal. Ahh my misspent youth.