I almost always use nylon control horns on my stuff. When using them it's a good idea to reinforce that mounting area to add strength, and also make the control surface more resistant to crushing when tightening the horns on final installation. Before the control surfaces get their silkspan I've got a routine for this reinforcement using 1/64" plywood and drywall spackling. These photos show reinforcing the elevator for the P-38, it uses two horns, one on each end.
First, I add a 1/64" plywood reinforcement piece to the horn mounting area using CA, doing both the top and bottom of the control surface. Next I'll put a sanding reference around the control surface using some low-tack blue tape. Then goop on the spackling, making sure it's thick enough to fill what will end up being the transition area from the plywood surface back down to the original control surface. It doesn't have to be pretty, just make sure you add enough of it to do the job.
Let the spackling dry overnight to make sure it's set up well all the way through. Then take a FLAT sanding tool of choice and slowly and carefully sand the spackling down to where it's level and even with the plywood, and the plywood is showing again completely. A sanding bar with 320 works best for this process. Then, start to angle your sanding bar down from the plywood to the original control surface, aiming at the blue tape reference line. Keep sanding at this angle until you've sanded all the way through the spackling right at the tape line. You want your sanding "goal" with all of this to be a straight and level surface from the edge of the plywood over and down to the tape reference line.
Once you have everything sanded to this point then CAREFULLY remove the blue tape, pulling it off by keeping it flat against the control surface and at an angle away from the spackling. After that the area should just need a minor touch up with some 600 or 800 to completely smooth the transition and have it all ready for silkspan.