Well, you could silver solder a small piece of steel to the top of the breech-plug just behind the barrel and file a notch into it. If you really didn't want to solder you could even drill and tap a hole in the same spot and screw a screw or threaded piece of steel in and then file a notch in the top of that.
Is there no trigger guard on this piece? That would not be something that I would do without. (And yes, I know about the Scottish, Caucasian and Near Eastern guns made without them.)
Here's a barn idea, instead of ramrod thimbles, how about just a couple of iron staples to hold the ramrod? that would be easy and severely plain.
What shape of stock are you going to use? I hope you will post pics of the finished project.