The normal method for a route or cavity is to fill it with wood as tightly as possible then use small amounts of a filler substance. The filler can be either commercial wood filler or a mixture of small wood chips and sawdust and glue or epoxy. The filler will allow smoothing it out, but too much filler tends to cause shrinking and cracking of the fill, hence the need to use something solid for the larger part of filling the route.
Again, it all depends on the size of the routes. :icon_thumright: