Question Is it possible to install tether anchors for third row in 2004 Mazda MPV?

I have a 2004 Mazda MPV with LATCH in the two captains' chairs but I would like to install my Diono Olympia in the middle position of the third row when my fifth child is born this summer (I have chosen this position because that seat has no head rest and only a lap belt. I figure it is safer to have a car seat there if I can get it installed well so my child who is there will have whiplash protection. Because my 4yo fits well in the Olympia, I thought putting him there would be good). There are no tether anchors for the third row. Is it possible for me to have someone bolt something inside the rear cargo area so I can use the top tether with the Diono Olympia?

Or if anyone has any other suggestions for five kids in a 2004 Mazda MPV I would love to hear them. My plans are/were:

newborn baby in an infant seat in a captains' chair (car seat yet to be purchased- I'm still not sure which one I'll be getting)
4 1/2 year old son 5-pt harnessed in Diono Olympia (he's 38 lb) in the middle of the back row if I can get a tether anchor
7 year old daughter 5-pt harnessed in Britax Frontier, preferably in a captains' chair as that seat is hard to install in the back (she's about 48 lb)
10 year old son in a low back booster in back row on one of the sides
13 year old daughter who just uses the vehicle seat belt also in back row on one of the sides

So getting the tether anchor seems like the best option as I'm not really all that comfortable with anyone sitting in just a lap belt in the back without a head rest.

Any and all suggestions welcome. If I can find someone to install a tether anchor, how do I make sure it's safe? Is it only okay to have a dealer do it or can someone else do it? I'm happy to have the dealer do it if they are willing, but if they won't how do I know who can do it safely? I know there are tether anchor kits for older MPVs- can I just buy one of those and put it anywhere, or how do I know if it is safe?

I live in the U.S. if that matters.

Wow- just got on with a question about 4 seats in my 2003 mpv. Are you sure your s doesn't have an anchor for the center rear seat- ours does it's just hard to find. You have to pull up the carpet in the trunk.

It isn't possible to get a dealer to do it because they don't make an official add on anchor. You can't use the retrofit for this.

You can talk to EZ-on about the possibility; they may be able to help you determine if there's an appropriate spot for retrofit or not if you ask about whether it is possible to install their heavy-duty tether anchor there (wew.ezonpro.com is their website.)

Another idea would be to pur a rear-facing child there; either the baby or the 4 year old if possible.