I'm not sure exactly what the OP is after, but I have capped stone and brick walls simply by cutting one line off a styrene sheet of a stone or brick pattern and gluing it to the top of the "wall". A bit of sandpaper and paint finish it off.
www.bostontype.com
If you are doing a random stone pattern, cut extruded foam to the shape you want and use a ball point pen to scribe the shapes of the stones.
Have fun,
Richard
The Chooch cast resin "cut stone" wall that I used on my layout as a retaining wall under and near a bridge was molded to be perfectly flat on top, which is prototypical for stone that has been cut with flat tops and bottoms, but my prototype had specially capped stone with the rough stone texture on top too. I assume this is what the OP is trying to capture.
How I did it is in my Frugal Modeler column in an issue of the NMRA Midwest Region Waybill, found here:
webwaybilfal12 (mwr-nmra.org)
http://www.mwr-nmra.org/mwr2016/mwr-images/waybillfiles/waybill20123fall.pdf
In brief, I used balsa wood strip of the same width as the Chooch casting which was "tortured" with knife cuts and also deep attacks with E L Moore's old friend, the hobbyist burning tool (a soldering iron would likely work too). I took care to cut evenly so each "stone's top" matched the stone piece under it. The thing about the burning tool, a/k/a burning pencil, is that it tends to seal the deep grain of balsa. That's why E L Moore used it so often.
Once done I also applied some diluted white glue to further seal the grain, before painting the entire wall, DullCoting it, and weathering it.
The article shows the prototype I was trying to capture as well as my results. If you have the balsa and have a burning tool it is certainly a frugal method, cheap enough that you can keep trying until you like the results.
Dave Nelson
If you want a plaster casting to match the wall casting, you could get a piece of C-channel styrene and use liquid latex rubber to create your own mold for the cap. It will take several thin layers of latex. Then you can cast the cap from Hydrocal.
It takes an iron man to play with a toy iron horse.
Which type of cap?
I use strip styrene, cut to fit. It can be painted if desired.
Mike Lehman
Urbana, IL
With foam.
Brent
"All of the world's problems are the result of the difference between how we think and how the world works."
How does one build a cap for a radom stone wall 1/87 scale?