A foamy? I'll watch for a new thread. Can you drop a link here?
The cardboard model follows The Template except there is no taper in the rear elevation. What that does might add some rudder effect. Today I'd probably use a Gothic arch, like this:
I think the tandem axle (rubber torsion trailing arms) with 10" wheels would add stability compared to a single axle with 15" wheels. And some horizontal fins might damp out any 'porpoising'.