I've had a rethink on how I'm going to route it through the undertray.
I'm going to pass it through right below the sway bar mount bracket. But, instead of drilling a 3" hole and sticking the tube through, I'm going to print a bracket. On the far side I'll add some BMW under trim screw clips. The bracket will pass through and insert into another bracket on the other side. Screws will pass through the outside bracket, then through the underbody panel, then into the far side bracket with attached screw clips. This should minimize the hassle for any project where the undertray has to come out, or where I need the ducting out of the wheel well for car work, and make replacement hose easier (and less of it required) if something happens to the hose in the wheel well.
Inside bracket:
Outside bracket:
3 screws will take everything apart, while leaving the hose clamped onto the bracket
