can't do a 10" free air. too small of a space to be considered free air. i've run the numbers, a few times, it comes in at a big ported box, but always run stuck at the uncontrolled venting of the 'box' no way to really tune that venting... at 6.9cu ft, it's almost the right size for a 'sealed' enclosure for a single 15".
get a box:
https://www.ebay.com/itm/11372440561...cAAOSwkFZcGlCj
i also wouldn't bother with 6x9's. the rear speaker locations are big open cavities that aren't easily sealed, so there won't be any improvement in audio response putting a set of 6x9's back there.
i run a 10" ultimax, previously a 10" peerless xxls. it works just fine.