HW30S would be my choice.
The HW95, as mentioned by Vernal, would make a very interesting option if the "galling" scares, er, scared you off the tiddler. The power output will be higher than required, though. However, if you're happy to open it up and fit a piston nose extension to reduce the power output combined with a weaker spring, this could be absolutely awesome. Fitting a weaker spring without shortening the stroke would make for a very lazy firing cycle, hence why I suggest the nose extension; available cheaply from Tinbum.
Open sight requirement would mean sourcing the HW85, as the 95 doesn't have them. Available from Sportsman (maybe elsewhere too?).