The metal rod is not always the best answer. I've seen trainers that have them... and the rod ripped through the wing when the wing snapped. (the rod was fine[&:])

The most durrable and cheapest trainer is probably the SPAD Debonair. www.spadtothebone.com Costs about $5 to $15 to build (depending on where you get the stuff) the airframe. If you buy the stuf... you end up buying enough to make 2 or 3 in order to get enough to make one... so plan on spending $45 and having material to make 2 replacement airplanes.

No metal in the wing structure... unless you decide to replace the spar with aluminum stock. (I've seen that done... its heavier but not really stronger in any way that matters...)