Or, you could simply buy a replacement from West Wight...

http://www.westwightpotter.com/

...maybe even upgrade to their 'Kick Up Beaching Rudder'?

The issue with Douglas Fir plywood - marine or otherwise - is that is
'checks' - or cracks along the outer ply layer. Water can ingress
along these cracks, and the rot process begins.

The solution is to cover the plywood with fiberglass (stops the
checking) and epoxy resin (stops the water). The application of these
materials should be considered carefully, meaning that additional
research into 'fiberglass and epoxy plywood encapsulation' should be
undertaken before starting.

Note that any screw / bolt holes in or through the rudder / tiller are
an area of concern.

Common practice (for bolts) is to drill an oversize hole, fill with a
thickened epoxy slurry, allow to cure, then re-drill with the
appropriate sized hole. In this way, water never gets to the wood.
For screws, much the same except allow the crew to penetrate the wood
for about 1/2 its length, i.e. 'drill oversize' for 1/2 the length of
the screw.

Anyway... good luck!

MW