How much bracing is needed for a sealed enclosure that will be about 7ft3 for an 18" subwoofer? And how do I go about bracing it? I will go with an Exodus Maelstrom or a Fi Q powered by a Behringer EP2500.
You could do as Mike said if you dont want to do this yourself. However, its really not that hard. You could goto a hardware store and get some mdf,show them the specs and ask them to cut it.
You're better off doing the glueing and putting in the screws on each panel yourself, as this would save u some $$$ of labour fees. But again, go get a quote from a cabinet maker and see how much they would charge you for a job like this.
Just to give you an idea (here in Aus), i built a similar design using a 18inch Mach 5 IXL 18.2.2 driver in a 107L box made out of 1inch MDF. I got a quote from a furniture builder and he quoted me $300 incl mdf and labour. One issue with this is to fit the cabinet into the car, which i knew wouldnt unless I had a van or pay more to get them to deliver.
I took the same specs and went to a hardware store and they charged me $80 for 1 1800mm x 1200mm sheet of 1 inch mdf and $1 per cut. Glue cost me $4. Clamps cost me $120 (I got a few). I also got a few tools, router, 18v drill and a circular saw ($260).
The question that you should ask yourself is if this build gonna be a 1 off build or not. I initially thought it was, but once you do this, the effort and satisfaction will drive to to test out other types of builds..ie: ported, LLT etc.. A close friend of mine was impressed with my sealed build, that he wants me to help him build 2, but he's put it on hold due to financial issues..
For sealed enclosures (or ported for that matter), depending upon how high they will be crossed over, I usually recommend bracing on all three panels at least every 7 " or so. For my personal designs, I usually am somewhere around every 5" or less. Always make sure the bracing ties all the panels together.