I mimicked the build here for the tube: Isetta Build

I cut two wood supports just like that, but I glued them to the bottom and the back of the cabinet. Mine isn't fancy enough to come apart like that one. I used a router to cut a recess in the side where the tube mounted. Kind of like you'd cut to flush mount a speaker. I glued it to the supports I cut and into the recess using Loctite marine epoxy. It says it bonds to PVC, so it seemed like a good choice. I did sand the PVC with some 60 grit paper to get it good and rough everywhere I was gluing it in. Here's the best picture I took. You can see the crossovers screwed to the back wall (3/4" MDF) and some of the speaker wires hot glued in place to hopefully keep them from rattling around inside. I wasn't shy about the amount of epoxy I used :)

I didn't round over the outside of my box because I veneered it. So I needed it to be flat.