A color washer around the posts near the entrance to the outlanes. Red might look good.

https://www.etsy.com/listing/210912288/washer-toss-pitching-washers

Pitching Washers

You could position a post same as the one at the back of the flippers to close the gap to the outlane as below.

Untitled-1 (resized).jpg

http://www.pbresource.com/hhouse/gtb-c17492.jpg
How bout something like this but only fastening with one stud?

Pinball Resource (resized).jpeg

You could play around with making your own wire form to mount around the post screw and below the plastic. Something like this:
Post wire (resized).jpg
Start with something soft like bailing wire and experiment with different shapes. Once you've found something you like you can make a more permanent wire form with piano or music wire available at PBR or Marco. Music wire is much harder to work with but has a nice shine and will hold its shape much better. Stainless steel wire is another option.

You might want to add a washer between the new wire form and the plastic above it to protect the plastic. You might also need a slightly longer screw. But, you get the idea...

/Mark

Since you have two posts on the lower edge of the violin plastic, you could run a stiff wire between them with a profile that narrows the gap, yet is fixed at both ends. It would be sturdier than the single post mount.
Post wire 2 (resized).jpg
Or maybe you could cut a piece of acrylic in that same shape that would mount on the posts just below the rubber. Since the posts are tapered, the rubber ring might hold it in place without any glue if you got the hole size just right. Or you could look for those two piece posts that are meant to have a clear plastic mounted at ball height and replace your existing posts with those.

If you can get your hands on some spare posts, you could drill a hole in the side and insert a deflector pin. PBR sells them: the part number is GTB-A10542+.

You’d want to put something on the tip to keep it from scratching the ball.

Pin (resized).jpg