The original plastic caps supplied in 1969 and onwards are, in my experience, the best way the seal the Holt Allen HA123, and the near identical Allen Brothers A123 drainage hole fittings. I think these caps have a degree of flexibility, combined with a slight oversizing, that produces a firm fit which is hard to match. Sadly these caps are no longer available (or at least, I've not found anyone who manufactures them, if you can suggest somebody, or a source, please contact Martin Egan).
Rubber bungs or corks are widely available, but are easily dislodged by accident (in particular the ones into the side tanks which are close to the helm's feet).
Below are some suggestions for the best alternatives to to the original plastic cap if you have Holt Allen HA123 fittings. Please note that the Allen Brothers A123 has a slightly larger diameter (we are talking tenths of a millimeter difference here), so it has different suggestions. Oddly, the drain hole fittings on Mirror 70447, which dates to 2006, before Allen Brothers was formed, are also slightly larger, so if you have post 1995 boat (the HA123 fittings I'm using are from 1969 & 1995 boats), you might want to proceed cautiously.
The key dimension for a plug/cap for the HA123 is the outside diameter (OD) of the plug just below the flange (i.e. the max OD). This needs to be somewhere between 15.7mm and 16.4mm, depending on the flexibility of the plug (the original caps have an OD of 16.4mm, but are quite flexible).
Tapered plastic plugs with large flange
This is what is meant by a tapered plug with large flange.
The best I have found so far are the Mocap TWF.560/.631YW1 which has a large OD (dimension B in the drawing above) of 16.0mm and a flange diameter of 24.6mm. It's made of Low-density polyethylene (LDPE). The plastic is quite thick, so these are not that flexible. Just a small amount of force is needed to seat the plug in the fitting as shown above. Minimum quantity from Mocap is 100 & there is a postage/handling charge as well. Maybe Class captains/Clubs could buy, and sell on to their Mirror fleets or give sets of 5 as prizes etc. Alternatively, you could opt for the nearly identical Essentra Components LF-11X (which is blue).
Barrel plugs
This is what is meant by a barrel plug with flange.
I've tried these, and the Mocap BPF200NA1 fits the HA123 well. It has an OD (dimension A in the drawing above) of 15.7mm. These have small flanges, so are hard to remove with your fingers (you need strong fingernails). Minimum quantity from Mocap is 100.
I'm going to try and check out a barrel plug from Vital Parts, sadly it also has a small flange, so will probably be hard to remove.
Silicone Cone Caps
Ron Vass is recommending the SCC007 Silicon Cap from Vital-Parts. I've not checked these out yet.
