Looking at some speed models at our local C/L club ( Barton MFC ) they are SOOOOOoooo simple you could scratch build one in one night.

You just need to find the aprox dimensions ( or read the rules re model dimensions on the BMFA rules web page )

They really are just an inboard wing - 2 x planks internally grooved to take the leadouts.
A simple plank fuse with bearers
A simple little tail plane

I think some actually call them a "3 plank design"

The bellcrank is usually mounted on a lil ply plate on the outboard side of the fuse ( no wing there )
A simple tank, sidewinder motor and pipe and off you go !

You could of course add a bit of spruce re-enforcement and / or carbon fibre...

Then you might consider a "fall away" piano wire launching dolly