Defroster Duct Mounting

My 58 did not come with a factory heater so I'm adding one. I got the plastic defroster duct and plenum from Walcks. How exactly does that defroster duct attach under the dash. I don't see any studs, so does is sheet metal screw from the bottom, the top, or bolt from top down or what. Any pics would be appreciated.

 

Originally sheet metal screw down through the cowl into a metal tab. The metal tab was attached to the cardboard duct. You can see the remnants of the cardboard on the tab in this pic...

 

Like Tim E says.

 

I can get you a pic if you'd like.

 

I haven't lowered the windshield recently, but I'm assuming there is space in that rubber seal so the screw heads don't create issues?

 

I haven't looked at the cowl seal lately, but I am pretty sure there are holes/recesses in them to make room for a multitude of things. They are fairly thick as well...when new at least.

 

If you zoom in on my pic you can see the windshield to hood seal with the holes for the defrost, and the 2 hex head bolts that hold the defrost tubing connecting.