Right on the money there, as if you use contact adhesive, for example two way tape, the moisture will eventually make it fall off, and if you use a superglue type product you will dry out the plastic and it will eventually crystalise.

Regards to getting someone to fit it, if you no longer have the part, it would be easier just to go to a wreckers/subaru parts dealership (whom ever you purchase it from), and either sucker them into doing it for you or just doing it yourself.

Dependant upon whats involved it may be easier to jack it up, remove the tyre and do it that way (if purchasing a new unit).

If you havent got anything for the part to slot into, use masking tape to secure it into position so you can ensure that it wont move, and remember that you shouldnt really drive the vehicle for at least 24hrs after securing the old part back on.