While there are probably a lot of manufactured boxes that will keep up with Labs in terms of SPL, there are not a lot of manufactured boxes that will match well. The SPL TD-1 and the Danley SH-50 are probably the best match, both of which are horn loaded to 80Hz or below. Most manufactures find it easier-cheaper to stick a fifteen in a box for bass-midbass either direct or in some bandpass configuration. In my opinion and experience this is not as good a match for the Labhorn as a true midbass horn.
For DIY I would go with the Fitz DR horns over the Xtro because of the expense, weight, and arrayability,(they are both difficult builds). I have never built or listened to an X-tro, so I can't comment on the sound. The Fitz horns require some eq to tame, but can be made to sound great, and definately do get loud.