Teardrops n Tiny Travel Trailers

Questions- while these are not exactly the same, the concepts should cross over. The question concerns how to attach items to the outside of the canvas covered shell. In a nutshell, can I glue them to the outside?
I'm considering options for a vent, Somewhat like a trolley top for a cover made from foam covered canvas. The whole assembly could be glued to the exterior of the trailer to keep moisture from entering the hole. The other thought is a drip rail, Can a person fashion a strip of foam to deflect water around the door and simply glue that to the exterior covered by canvas.
I know I could include hard points within the phone as attachment points. However I don't want to. Additionally I will have plywood interiors so that I could use screws if needed.
Thank you In advance for what will undoubtedly be entertaining and potentially conflicting responses

Rog , easy to glue foam to foam and then cover it with canvas or whatever... If there’s no stress on the part , like a drip rail , etc.that should be sufficient ... You can always make emergency quickie hard points by drilling a hole in the outside of the foam and inserting a suitable size dowel , drilled for a screw... Then a simple round patch over that and you can attach anything you want ... within reason ...

I thought about the drop rail too... After I had fiberglassed my sides. I am sure (as proven by the caps on the wall to roof joint) that you could create a foam rail, cover it with whatever material you choose and either glue or epoxy it in place and it will hold just fine. As far as anchor points, dowel works great. You can also make the anchor point stronger by tapering the hole to a larger size at it's bottom and fill the hole with epoxy and then insert the dowel. Kinda like how pier footings are pre-cast.

If you need a heavier duty attachment point , you could do something like this , which spreads the load out over the foam ...