Step 1: Design Thoughts
Points to consider:
Streamlined for aerodynamic considerations
Blend in with the fuselage
Have some vent holes for oncoming air to cool the motor.
In my case Im using an electric speed 400 which can get hot if totally enclosed, so I need some air flow past the prop and an escape outlet in the fuselage to prevent pressure buildup, which might cause the covering to balloon.
Step 2: The Shrink Process
Start by cutting off the bottom and inserting the wood plug.
I'm using a 2L milk bottle for a demo, so its way too big for the plug, but its just to show the procedure.
Next shrink the bottom of the bottle first to trap the plug inside. Then do the neck area, here I've cut off the threaded part to make the shrinking neater and easier.
One you're happy with the entire look its time to let it cool and the trim the waste so as to pop the mold off the plug.
Step 3: Time for the Test Plastic
The bottle of choice is a 500ml PET type. First I made a wooden plug to shrink the bottle around and then I did a test shrink with clear coke bottle.
Also, I made the plug slightly larger than the fuselage so that it could be slipped on and taped in place.
Some practice is needed with the heat gun to stop bubbling and white-out.
I also gave the wood plug a few wax coats to make it easier to pull the mold off.
Step 4: Final Choice and Shrink
Fixing the cowl on with sellotape is easy to remove if needed but otherwise sticks well.
There was lots of standing back and checking the appearance and of course admiring the handy work. :)
When it looks right and feels right then its time to stop.
Incidentally the plane is a 440 gram park flyer.
Speed 400 aerobatic plans at Flymodels UK