Personally I would forget the idea of a small fixed blade knife, they probably were not used much in this way. A better choice if you wanted a smaller knife would be a reproduction Barlow folding knife, or a friction folder. This is what I use.
What you want then is a common knife that is correct for your chosen time, location and station. There really were no "patch knives". I would search put a common butcher or altered table knife of the period. That said, I can document antler handled knives on the straps of hunting bags to the 1820s
A penny knife should make a good patch knife, and they are light in weight. Those offered by Clay Smith look very good, and are more than fairly priced. When I make them, mine sell for over twice that. Clays' penny knives look like a very good deal.