Whole House Surge Protector connection
You'll have to install it in what you put in for a replacement subpanel, unless you want to do concrete work

Here's the rub -- that meter-main of yours is basically a boat anchor.  You can't realistically replace it (conduits set into concrete act as a practical bar to that even if you got all your other ducks in the row with the utility pulling power, getting the permit needed, and finding a replacement that'd fit the space available), and you can't expand from it either as it only has room for that one main breaker, never mind having to screw around with the concrete just to get at a usable knockout.

The good news is that installing the surge protector into the subpanel will still work well enough -- it just needs to be near the breaker feeding it, and that breaker should be as close to the primary (would say "main" but that's not right in a setup like yours) set of bussing as possible.

If you're up for chiseling out concrete (carefully!)

In order to install the SPD at the meter-main, you'll have to:


Chisel away enough concrete to access a 1/2" KO on the lower right of the box and a notch extending about 6" out to the right from that KO, then make the KO in the lower right of the box using a KO punch set since your box doesn't have a twistout there.
Fit a 1/2" prefab RMC elbow to the knockout with a locknut so that the free end points out (beats field bending, huh?).
Install a NEMA 3R "spa panel" enclosure (BR24L70RP if you're staying in BR) to the concrete behind it so that the prefab elbow goes in the twistout on the back of the enclosure, and fit a ground bar (GBK5 for BR) into the space provided in the enclosure.  Make sure the bonding screw or strap has been removed from this panel.
Disconnect the existing wires from the breaker lugs and neutral lug and land them on individual 3-way insulated setscrew connectors ("Polaris connectors"), torquing the connector lugs to spec.
Use short lengths of aluminum wire matching the existing feeder wire to pigtail the connectors to the existing lugs, torquing all connections to spec.
Take 8AWG THWN (black/black/white) and pull it through the short elbow, attaching them at one end to the corresponding mechanical splices and at the other end to the appropriate lugs in the new mini-subpanel, torquing the connections to spec.  Why 8AWG? It can handle 50A at 75&deg;C, 3 8AWG wires will fit down a 1/2" conduit, and we're legal to use this because we are following the 240.21(B)(1) case of the NEC feeder tap rules.  Note that the feeder tap rules require the wire to be in conduit -- a cable wiring method simply won't fly here.
Install a 50A 2-pole breaker (BR250 for BR) into the spa panel enclosure.
Fit the CHSPT2ULTRA into the knockout on the bottom of the spa panel.  Land the hot wires on the breaker lugs, the neutral wire on the existing neutral bar, and the ground wire on the ground bar you fitted (it's fine that it's the only wire there).


(And this is why I recommend installing the surge protector into the new subpanel -- it's far easier that way.)