There are several methods to achieve a longer servo arm for the HS-55's. One way that is very easy is to make an extension (as long as you need) out of 1/16" birch ply.

Make it large enough in the center to fit over the standard X-arm that comes with the servo and then extend the arm portion to the required length. Be sure to drill a hole in the center so you can attach the arm back on to the servo once your assembly is complete. You can attach the ply to the X-arm with epoxy or medium CA. Just keep the glue out of the center hole.

A picture is worth a thousand words. Hope this helps!