The standard location is the passenger footwell, preferably towards the front. You can mount it underneath with three holes or cut an oblong hole and mount it inside the car so that the "block" can be dropped in from inside, with a strap or perspex plate over it. The latter allows you to see it's serial number easily.
Mounting it underneath means that small stones, gravel and dust gets in, making it difficult to get out of the cradle. Also, you don't want water to get into the cradle.
Mounting the Dorian in the spoiler is a "no-no", as I have heard of at least one story where a competitor lost a spoiler in a race and then did not get the spoiler back until the end of the meeting so he had no timing mechanism.
Re: Dorian location
It is in the manual, for saloons underside of the front passange footwell, local modifications are allowed
Front spoiler would actually be cheating