Did you buy your engine new, or was it part of a package deal? If you got it new you should have the washer and nut shown in the picture and you should use it instead of the spinner adapter that we've seen in the pictures you have posted. If you do not have the original washer and nut, you have to try to deal with what you have.

All of the pictures you have posted show a socket head screw in the spinner adapter nut. Since you wrote that you have a Dubro nylon spinner you need to take the socket head screw out and put it in your toolbox.

I sort of hate to suggest this, but you may need to use pliers to keep the engine from turning while you get the prop snug. Once it is snug, friction should hold everything still while you really tighten it.

Use some rubber strips, cloth, or whatever you have to protect the "flywheel" from the pliers.

You'll need something like the ones pictured to get a large enough jaw opening to grab the "flywheel." Be sure to protect it from the pliers.