Just a thought...unsolicited.....if I were looking for a cabinet in which to put a vintage JEnsen...or any speaker, really....I am looking for a solid cab. A thin fiberboard cab doesn't allow a speaker to yield its best sonic results. Thick fiberboard...MDF....can make a decent cab. TEh only time I would be looking for a thini fiberboard cab would be if I were trying to emulate the cheap Danelectro/Silvertone cab sonics. 'Light' doesn't mean good sonics when it comes to amplification.
Also, you aren't really looking for a chassis, either, are you? You are wanting a speaker cab...or an empty amp cab, right?
Avatar sells some very inexpensive speaker cabs, ime.