I've seen some people with wooden boats string a length of
light chain from one end of the boat to the other..threaded
through the "limber holes."
A spring, or piece of bungee cord or similar elastic, attached at either and dead ended, will hold it in place.
Every now and then, reach down and grab the chain at any place along its length and give it a couple of tugs to keep the "limber holes" clear of debris
NB