Plenty of people have done that....my impression is that by the time they are done buying lumber, fasteners, and insulation they could have found a cheap chest freezer off of Craigslist for less time/money. However, some people like the DIY at any cost and for a custom solution.
Speaking of which, what are your plans for keeping it cool? If your basement is 70 your fermenting beer is going to be 75-80 which is too warm for most ales. It will probably be even higher in an insulated box.