Nice cabinet, you are on the right path. You are learning from your mistakes. I have built 4 cabinets already and I learn lots from each build as I have made more mistakes than you can imagine.

If you are still looking to hang your vent (I'm guessing to reduce noise) try using small bungee cords it worked for me on one of my cabinets. First I used rubberbands, but then switched to bungee cords when the rubber bands broke. Sorry I only have a picture of the rubberbands.

Good Luck!