4X10s

so i got these 2 sorta old but brand new 4X10s and i was wondering were the best place to mount them would be... my current setup is 2 6.5 s 2 6X9s and im throwing 2 12"s in soon i have a 1998 chrysler cirrus
would making a custom box be a good idea or just fabricate them in the trunk somewhere...

Why do you need anothers set of speakers? Especially 4x10's. I wouldn't waste money on putting a ton of speakers on my car. But If you gotta have them just put them wherever your want....just don't wire them with your other speakers in paralell to the headunit. HU's can only go down to 4 ohms.