In the Gunsmith of Grenville County, George Schreyer's patchbox release system is shown in great detail.
He used a pushrod that was activated on the top (comb line) of the buttplate, that was fastened into the the latch spring (rather than simply pushing it).
The latch spring was peened into place on the butt plate as well and then groves were cut in the butt stock to allow the whole shebang to be fastened as one unit (so push rod and latch were integral to the butt plate itself).
Really difficult to explain but don't want to scan and post the pic from book (for copyright reasons).
(if you can beg/borrow a copy of the book it can be found on page 284 of the "reprinted" edition)
Personally, I would go with the patchbox release kit that MBS sells for under 10 bucks - unless someone pulled your butt plate you could never tell it wasn't "the way George did it"