These Neutrik NAUSB - USB Feedthrough Chassis Socket are pretty good. You just need to drill a hole for the barell of the body and then you just screw it to your panel.

I'm looking for some low profile 3.5mm panel mount jacks myself at the moment.
I have seen these panel mount cables:
You can mount them into panels up to 2.5mm thick, I need to mount them in 3mm acrylic so I may just get away with it.
Another solution may be to use a panel mount jack something like this:

You/I could then just wire a 3.5mm cable to the terminals.
Depending on your application, another solution may just be to get a 3.5mm extension cable (kinda like the second pic) and then just drill a hole in your panel to suit the OD and glue it in place. I'm using a thin acrylic panel so that last method isn't really suitable, plus I want something clean and I also hate having to glue anything unless I absolutely have to.