Wooden rail replacement plugs

We have these wooden track elements which have "spherical connectors"; they came out after a while - I tried to glue one back in but it came out after a while - and I never really liked them - because they give more free space when connected....

I've designed this plug to fix them...and it seems to work (actually this is the 3. version of it - but the earlier ones were coming apart after some use...so they are not really usefull)

There are 3 sizes of the plug (5.5,6,6.5) ; I printed a bunc