thats a fine box design. like he said move that brace back a bit and also dril some nice sized holes into it as well as add another brace just like that one... pvc and abs both have a resonance freq. that is close to what a sub can produce so you could get some vibrations... a good idea also is to put a very thick and fluffy but very breathable material into the port or build a small sectionto jam it into... this wil slow down the air enough to keep the sounds in the box and up db a bit.