One thing we did was to put a platenut on the forward elevator horn, so that if we need to add weight later, we can attach it via a screw into the platenut (washers, etc.). I've heard of people adding weight to a completed elevator w/ fiberglass tips in place by mixing epoxy w/ lead shot and pouring it into the tip (something we used to do to nosecones for high powered rockets to get CG and CP correct), but I'd rather be able to make sure the weight is secured w/ a screw.