A method that some might think is a shade too permanent is to simply "stick" the motor into the hull, Fig 5. Again some packing is usually needed to get the motor and propeller shafts aligned.
As for the "glue", I agree that an adhesive like epoxy is a rather risky since in event of the motor needing to be replaced, then serious damage to the model is likely. However, using something like Silicone sealant on clean surfaces can produce a firm bond to hold the motor in place yet still allow its removal with careful cutting and modest force. A latex based adhesive has also been used successfully.
A recent model used one of those "sticks everything" tubes of adhesive (actually called "STIXALL" too!) to great effect. The bond was more than strong enough but I could still remove the motors without ruining the model. This method was also used in a small destroyer model with twin motors where there was no room for any other way of securing the motors side by side, Fig 6.
Yes, all these methods of mount the motor have been illustrated with figures based on a hull with a flat bottom. But, with a little thought they could be used on other types.
Glynn Guest