Alright, here is a Walther's Great Circus series stock/elephant car that I picked up used. I've chiseled off the cast in grabs and plan to open up the vents. I'll install some micro screen in them to make the see-thru and add wire grabs to kind of bump up the looks of the car. On the train I'm building there will be 2 of these type of car as well as 2 or 3 of the horse type stock cars.
Here the car has has a wire grab installed on the lower left side. Note the cast vent openings on either sides of the door opening.
Using the old placement of grabs, I drilled openings for the wire ones.
I drilled starter holes in the cast vents.
Then, using a modelers knife, I carefully cut to the corners to start removing the vented area.
When the plastic was cut away, this is how things looked.
Next is using a file to get all the edges clean and smooth.
And when the windows have been cut out, things look like this. The car has identical openings on the other side.
It may be a week or so before more pictures are posted as the next car is done the same way. When done, these cars will be circus yellow with red roofs and letterboards.