In the past, I used one of those metal buckles along with the little red
switch on the back of the seat belt to get the seat very tight. I used
the metal buckle because the red plastic part looks a bit flimsy and at
least one person here reported there's broke. Basically, the metal
buckle/red switch converts the seatbelt to work like a lap belt so it
stays tight at a fixed length. The most important thing is that the
seat belt becomes fixed in length and can't be pulled out at the lap.

I use the middle seat since in a side impact there is only about 6 inches
between you and the car hitting you (that is why the center is safest).
Think about how much "car" is between occupants and the hood and you
see why side-impacts are so bad. The center position is a bit narrow and
may not work well with some car seats.

The car's owners manual should have some info on the rear tether along
with proper use of that red switch on the back. In Canada, tethers
are required by law so the manual covers them. Canadian models are
delivered with the tether kit (so I hear). A Volvo dealer should be able
to order a tether kit for a car sold in the U.S. market. I confess I
never used the rear tethers since you also had to call the seat
manufacturer to get their kit (unless you purchased the seat in
Canada). Seats typically have an 800 number stamped on them somewhere.
I think the seat manufacturer's will send them for free in some cases.