Firstly, it would be much easier to buy a tuned pipe. The pipes for the duratrax engines are very cheap, myself I am a Nitro R/C noob, but I picked up a Duratrax pipe for my kyosho engine for just 17.99US with manifold, coupler and mounting hardware.
Now I have no dramas and am happy with the power output. Its not huge, but its more than enough.
A similar one can be bought for the torq .21.
The aftermarket pipes are generally dyno tuned and tested, so they would produce the best results.
Now, if you still want to make one....go ahead.
Build 2 or 3 different pipes, but base the design on a production pipe.
Persoanlly I can't see the point, but I am an obsessive compulsive tinkerer, and can see your curiosities.
Even if it turns out to be a pointless exercise, you will learn a lot. You may also make some great pipes, you could use the duratrax manifold for different pipes.