We had wooden Marvin double hung tilt-pac windows installed but all without screens. However, there aren't any screen tracks. On the exterior trim, there is a groove/corner on three sides that would fit a screen (~36x54.5) , but it would have to be secured with hardware somehow. And this trim corner does not extend to the bottom sill - rather, the bottom sill is sloped down and smooth. I attached a picture of the lower corner to see where the screen would fit - I am pointing to the relevant corner where I thought I could fit in the screen.

How do I secure a screen to the trim without a screen track and without letting water into the trim/sill? I am worried about screwing things into the wood and letting water in.

[![window sill and trim area to fit a screen](https://i.stack.imgur.com/jFQTX.jpg)](https://i.stack.imgur.com/jFQTX.jpg)
['https://diy.stackexchange.com/questions/170161', 'https://diy.stackexchange.com', 'https://diy.stackexchange.com/users/89457/']
It does look like that a screen could fit where you're pointing. 

I've included a picture of some screen hangers and a lock.[![enter image description here](https://i.stack.imgur.com/KLBnJ.jpg)](https://i.stack.imgur.com/KLBnJ.jpg) The hangers would go on the top of the screen and trim allowing the screen to be lifted into your trim track. The locking device would be screwed into the center of the sill and mounted to hold the screen securely into the trim. Any water that accumulated during rain would be able to run under the screen. The hardware would be permanently attacked to the upper frame, screen and sill so it could be caulked and sealed to prevent any water from migrating into the wood.
Typical approach for modern screens/windows (as Jack has shown the very vintage approach) is that the screen has a frame (typically aluminum) that has springs on one side or two sides so it can be fitted into the slot by compressing the springs, and when released is held in the slot by spring pressure.

The sill is "trackless" so it won't trap water and create a problem. The screen frame being held in by the side grooves seals it well enough if it's properly fitted.