Boom for low bucks

Here's what it says in the Infinity manual:  "Bandpass enclosures often provide the most output available from any amplifier and subwoofer combination at the expense of sonic accuracy."  With that in mind, a bandpass enclosure is one of the most difficult to build...and get a reasonably good-sounding subwoofer.  If you are providing 200 to 250 watts at the 4 ohm impedance load that sub will provide, a small sealed box would be the best route for sound quality and mostly effortless construction.  With almost any subwoofer that calls for a 1 or 1.25 cu. ft. volume, you can easily go higher without harm...and usually get better low end response too.

Those cabinets for the Thrusters are most likely 5/8" MDF, so you could use those with a second layer of MDF or plywood.  I would probably use 1/4" exterior grade plywood to beef them up, using glue and screws across the whole contact area.  You can sometimes find it in 4 X 4 sheets, which will save you some construction costs.  And you can screw a square of 5/8" MDF (old shelving or pre assembled cabinetry lying around the basement should provide this) over the passive radiator hole on the inside....and no one will be the wiser.

Now I know why you guys call it Fleabay...

I bought my Infinity from a seller claiming it was "Brand New 2005" and when it arrived it had been damaged.  The outer rim was bent probably a quarter inch.  Saved almost $10 though whoopee!!!