1/ Width at top to be wide enough to take shoulder harness.

It looks like it will need to be 6" higher, I.e. 30" high.

 2/ Bottom to line up with top of rear supports (adjusted by bolt & spring plate) and or half way down tube, or all the way down to the bottom of the tube, with straight sides, or down below tube for oil reservoir.

 3/ There will be a smaller "backboard" on the lower half of the tube.

 4/ The lower side edge of the seat should have a small radius, say 3/4" inside and 1-1/4" outside. For appearance, because it will probably look better if there are no hard edges, or corners, on the helicopter.

 5/ The lifting location on the power train is the synchro box, since that is where the tapered bearings are. There fore put more strength in the fuselage backboard at this location.

 6/ Consider slight curve on back of backboard. It will look better, be a little better aerodynamically and not be a flat surface that might have a slight warp.

 7/ If mold for backboard and sides and bottom is one then it should probably be a split mold. This will allow shapes to be done in side or bottom. Note requirement for minimum of 5 degrees draft. 

8/ Shape, particularly in front view, should be suitable for future canopy.

For backboard 24" high, 18" inside width & 4.5" lower inside radius.

Sideboard to be approximately 17" deep. 

Circumference of lower corner.

(4.5" radius * 2 * 22 / 7) / 4 = 7.0714" 

Circumference of circle based on length of side.

24.4182 * 2 * 22 / 7 = 153.4858"

/ 360 = 0.4263" per degree.

 Arc of curve.

7.0714 / 0.4263 = 16.5879 degrees. 

Circumference of lower corner.

(5" radius * 2 * 22 / 7) / 4 = 7.8571"

 Circumference of circle based on length of side.

27.1315 * 2 * 22 / 7 = 170.5408"

/ 360 = 0.4737" per degree.

 Arc of curve.

7.8571 / 0.4737 = 16.5858 degrees.