I am putting together a Great Planes Reactor 160 with a DA-50. I would like to laminate the firewall for strengthening. What is the best way to do this?

Epoxy a plywood copy of the firewall to the original and brace it with tri-stock

Use GOOD ply not lite ply. ENJOY !!! Red

if you could incorporate angled aluminum bracing, and bolt them in with lock nuts, that would help too... ive had tri stock bracing split on me on my big gassers....

http://www.flickr.com/photos/31850530@N08/5669007974/ it's not the neatest looking work, but it has held up for years of somewhat extreme flying

It's just two pieces of aircraft ply and you epoxy them together cross grain. I know a fellow that likes to put a piece of carbon fiber between the two pieces but it isn't necessary.
To date I have never had a fire wall break either sandwiched or just one piece of ply. Bracing with tri stock as Mike mentioned along the sides of the fuse and fire wall is the key to keeping the fire wall from coming loose.

Carbon and Balsa is your friend... A carbon balsa firewall vacuum laminated with a soft mount is plenty strong and very lite