My Parker 21 lives in Christchurch so the bottom front corner of the rudder blade frequently scrapes along the bottom if I forget to pull it up and change to my "harbour rudder" (see other posts!)
When it wore through the plastic I sawed off all the part that showed damage and fashioned a similar shaped piece out of a lump of solid polythene (?) that I happened to have. This is fixed on to replace the cut off part with stainless screws and a liberal dose of Sikaflex or similar to seal the join.
I usually paint the whole rudder each season and the paint rubs off the plastic early in the season but it has withstood many contacts with the bottom including old mooring blocks and similar with no damage.