bracing is non important with a single 12, around 1 cubic foot, and 450w of power.
if the wood is free, use it. make sure its not crappy particle board tho, if it is i would just spend $16 on a sheet of MDF (medium density fiberboard) and see if you can use the machine to cut it. should not have to change anything, possibly just the thickness on the computer controller.
box of any size\shape is fine. do not worry about having a 'symmetrical' box as the speaker inside will displace any standing wave enough that it is a moot point, that is unless you plan on competing in the iasca finals lol. even then, i doubt it would be a factor.
that is nitpicking. and if its that much of a burden on your soul, just make some corner inserts or use some polyfill. *shrug*
so just calculate 1~1.5 cubic feet internally, and double layer your wood. and if i were you - since you have the tools, recess your speaker into the box by cutting the second layer of the speaker hole your baffle to the outer diameter of the speaker, and the first layer will be the mounting (read: inner diameter) of your speaker.
it will look nicer that way //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
otherwise, do whatever you like. it should be a very simple project.
and last but not least, USE "Type I PVA Glue"
as tests have proven that it has one of the strongest bonds for wood when not submerged in water lol.
titebond III is a type 1 PVA glue.
http://www.amazon.com/Franklin-International-Titebond-16-Ounces-1414/dp/B0002YQ3KA