RE: NITRO EXTRA 300S
I assume you are talking about the .60 Extra 300S.
I had no problem getting the cowling onto that plane, however it is a tight fit.
The first time I did it, it appeared as if it would not fit.
You'll find that in order to install the muffler through the muffler tunnel at the bottom you'll need to cut out some of the cowl.
When you do this the sides will expand, allowing the cowl to slide in place.
If the cowl were originally made larger there would be far too much of a gap.
BTW: Use a .75 engine on that plane for great performance.
I have a SuperTigre .60 on mine and I wish I had installed the .75, though I am not unhappy with the plane at all.