RE: .15 sized pylon racers
The 1/4 midgets were fragile and not designed for a rear exhaust + pipe.
In your position I would scale down a Weston Magnum - wing flat plate 3/16ths balsa - upright engine and either fit twin fins or a V tail. Should be well mental.
Incidentally I had a MDS 15 great little engine 24,000 +rpm on a toothpick but it is noisy really really noisy. Hope your club does not have a noise limit because it won't pass.