I bought 50 and then 100 more of these plastic plugs from Maud at Amel, La Rochelle.  I sent her a picture of them, as well as my SM boat number.  The ones I received from her fit snugly without alteration, and without the need to cement them in place.  Originally when I bought the boat there had been some spare plugs on board from the previous owner(s) that were too small, and I had to use a silicone sealant to hold them in place.  Best to measure the size of your holes to get an exact fit, and get quite a few spares.  They only last a couple of years in my experience.