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Invictus: (adj.) Incapable of being conquered, overcome, or subdued. Invincible.
Synonyms: unbeatable, unconquerable,  invulnerable, unshakable, indomitable,
impregnable, unassailable, insuperable, indestructible, supreme, insurmountable,
irrepressible
Invictus Mass Loaded Transmission Line enclosure

The Invictus MLTL has a modeled f3 down into the mid 20's and
an f6 of 20 Hz. Of course a single 7" driver, even one as capable
as the 8531 can only play so loud. Most will find it adequate for
serious listening, but those looking to recreate rock concert
levels may want to choose a different design. This is not to say it
cannot play loudly. I've yet to hear the Invictus complain with
whatever I've thrown at it.
MLTL Tower:

The prototype enclosures were made of 3/4" MDF and braced
with a single vertical ladder brace tying the sides together and
breaking up the two large side panels into 2 non-equal sections. I
also added two braces across the front panel.
The port location is up to you, as long as the inner port flare is
centered 4" from the inside bottom panel.    
Tips and tricks:

While I did not do so, certainly it would not be unreasonable to
add a couple of broomstick braces from the center of the two
front braces to the rear panel, although the 'knuckle rap test' did
not indicate any signs of undue enclosure resonance. the
finished cab will likely have a solid wood front baffle for a total
baffle thickness of 1.5".
I did not design a near wall version of the Invictus, and
suggested placement is to have their front baffles between 2 and
3 feet from the front wall. This doesn't mean they can't be placed
closer. The sealed versions will be more tolerant of near wall
placement than the MLTL, although the bass in the MLTL
doesn't sound too out of proportion. Certainly stuffing the ports
with foam is an alternate solution, and provides more design
placement flexibility.
Links to:

Main / Design Goals

Driver Selection

Sealed tower Cabinet construction
Sealed Bookshelf cabinet construction
MLTL Cabinet Construction

Crossover Design

Measurements
More Measurements

Listening Impressions
A two way design using
top shelf drivers
3 different enclosure variations
Cut List:
Since one of my local lumberyards has a panel saw, I have them cut the 49" x 97" sheets of MDF into more manageable sizes. The cuts below will allow
sufficient overage to square up the pieces back home. A pair of MLTL's take a little over one sheet.