Used non-Olympus 43.5mm skylight or UV filter in very good condition.
No scratches or fungus. Filter rim is without dents and will offer good protection for your Trip 35 - The filter rim of the Trip 35 is a little vulnerable to knocks, being right at the front of the camera.
Skylight filters will reduce glare and bluish tint of daylight. The UV filter reduces glare and haze. They both protect the front lens element of the camera.
These filters have a 43.5mm filter thread on the front, so you can also attach screw-on hoods and other filters. They are deep enough so as not to interfere with the focus on the Trip as you focus.