Safety Note: Do not melt lead or otherwise work with it without wearing safety glasses and a respirator and do all lead work in a well ventilated area.
The plans and Beartracks both mention balancing the elevators with lead and, when the word “balance” is used, that means putting enough lead in the balance area (the part sticking ahead of the hinge line) so that, when the surface has paint and fabric on it, it will balance horizontally across the hinge line.
There is a little guess work here because you can’t balance the surface when it has the fabric and paint on it, so you have to make an educated guess as to how much fabric and paint weight will have to be balanced. See Bob’s Bearhawk Book, pg.29 for more detail. This is done by folding up the approximate amount of fabric that will be used to cover the tail and laying it in approximately the center of the elevator surface. At the same time, you’ll have to add a little weight on the fabric to simulate the paint/finish. Fortunately, the tail doesn’t have to be balanced exactly. Casting the weights isn’t as complicated as it would first appear because you are going to actually cast them in position on the tail by making up some crude forms and dams to contain the lead. A word about lead: its fumes are dangerous. Only work outdoors and preferably with enough of a breeze to keep the fumes away from your sensitive nose. Also, don’t underestimate the energy available in molten lead! It will readily start a fire if allowed to contact flammable material like wood or vegetation. You must use a respirator, heavy gloves, and safety goggles, but a full face shield is better.
Some builders have been able to get lead weights from tire shops. If you use tire weights, realize that some are zinc, and you don’t want those in your mix. If you can’t tell the difference from handling them, hit the weight with a hammer, and see how soft it is. The softer weights will be lead. Don’t worry about the steel clips, they will float up to the top once the lead melts. You’ll need a small cast iron pot or ladle to melt them in, although a coffee can and a pair of vice grips will work. A long, feathery flame on a welding or propane torch is all you need to melt them. Just play the flame directly on the lead and keep your face away as all sorts of fumes come up as the oil and dirt on the weights gets cooked. Make sure your weights are dry so they don’t generate steam that can cause the lead to pop.
When the lead melts, the steel attach hooks and all of the impurities will float to the top and you can easily skim them off (it’s called “dross”), then you have lead that is ready to cast with. If this process isn’t going as you’d expect, watch some of the many Youtube videos on the subject. You’re going to cast the lead right in place on the elevator so your mold is going to be built around the tail and it’s really pretty simple. First drill a few holes in the tabs welded into the balance area to give something for the lead to hold onto. You might even put a screw through the tabs but make sure it doesn’t protrude above the surface of the lead. Also, run a few screws into the tubing and let them protrude. You can also run a couple screws through the rib surface. This is all to give the lead something really solid to flow around and lock it in place.
Next, procure your mold pieces. Some builders have used thin plywood, but realize that the plywood is going to leave behind a rough texture that matches the grain, and it is going to scorch and smoke. You’ll probably want to remove this texture before covering. Another option is to use ¼” aluminum plates. Clamp your pieces to the upper and lower part of the area that will receive the lead. That forms the surfaces of your mold.
If you are using aluminum plates, preheat them by playing the flame of the propane torch over the plates just before you pour. If you are heating with an acetylene torch, be careful not to burn a hole in the aluminum. The flame temperature for OA is around 5000 degrees C, for propane it is around 2000 degrees C. The melting point of most aluminum is on the order of 660 degrees C, and lead is 327.
Now, melt your lead and pour it into your makeshift mold. Once it cools, you can rough shape it with a cabinet rasp and 60 grip sand paper. Clean the rasp often as it’ll load up very quickly.
An alternative to casting is to mix a slurry of lead shot and epoxy. This will require a larger volume for the same weight but isn’t hazardous to your health.
Regardless of the particulars, be positive that the lead weight is not going to come loose after you have covered the elevator.
Now, install your elevator on the horizontal stab making certain the hinges are lubricated (light oil) and free. Put your dummy fabric and paint weights on the bare elevator and see if it appears to be balanced.
It’s probably not but it’ll be close and, if you intentionally cast on the heavy side, then you can balance it by gradually drilling weight out.

Here are three photos of the Model B version of the process from builder John Reddick: