Hello
This is not a good place to put speakers. As your ears are not in your feet.
Manufacturers ofen put mid range speakers in the front doors as they are too big to go elsewhere. To get round this they use component speakers which means a seperate tweeter speaker is mounted in a position pointing more towards or closer to your ears. Ideally the main speaker and its tweeter do not want to be too far from each other.
You could copy this practice for the rears as i'm sure people have done.
Is there a reason why you are not wanting to put them on the parcel shelf, such as having them on display etc.