mud flap...

Hi guys, anybody know how these are fixed/screwed on???
Both pieces are threaded, but I have no screws. Plus there is not a lot of space between the mud guard and the tyre.
I could put a flat headed bolt in from the inside but then I will have an ugly nut on the outside...
Any ideas????

 

Pop rivets, didnt want to try that, thought the plastic might split. Think I will look at the bolt option again....

 

Yeah, thought about them, but just worried if they will hold good enough. Dont want the thing coming off and going in between tyre and mud gaurd...