Eric - the "Loft" commented about 1 hour ago Ross, here's a though for you. What about using 2 long strips of say Oak 3/8" square with a block say about 1' long at each end for the strength glued to the longer strips. It will give you an opening in the center and add the strength. (Hard to explain in works).
I made adjustable stop block for the Kumiko Jig Block like that, they slide nicely in the dado and a screw with washer locks them in place. mine were 1/4" square here.
Eric - yeah, I definitely think if that top section of the plunger piece was 1" long, it would have resisted cracking much better. Maybe even a spline would work. Although, with the plunger having to be thinner than the center rail so it can slide (I think it ended up about 0.217" thick), the spline couldn't be very thick. Even a thin spline might be enough, though.
