That panel there is something some of us call a "Ranch Panel". It has a (typically) 200A main breaker, 4-12 breaker spaces, and then "Thru Lugs" onward to allow the full breaker ampacity to be carried onward to other points of use.
Must be installed past the main disconnect
To install it before the main disconnect, two things would need to be true: First, your panel would need to provide auxiliary lugs for that. I can't see for sure, but since this is a recent feature, I think yours does not. And second, your SPD would need to be a Type 1, designed to be installed like that.
That one is is a Type 2, which can only be installed after the main disconnect. And the panel doesn't appear to provide special lugs for that. Those bolts connecting the main breaker adapter to the main bus are not suitable unless the panel instructions and labeling say otherwise - which I doubt.
The surge can share a breaker, though
And this may be an answer here. You need to use a legal method to tap or splice: don't "slip the extra wire under the lug", don't piggyback terminals contrary to instructions, and definitely don't do this LOL
You better be laughing.
Given what's in your panel, I would get some black #6 THWN wire and pigtail onto one of the 60A breakers. Two #6 and a #12 should fit fine onto one of those very large wire nuts.
Is this really the right place for a surge suppressor, though?
Most people, if their radon detector goes off, they say "I must have radon" (reasonable). But if their GFCI trips, they say "My GFCI must be broken!" (say what??) It's true, you know I'm right. In a similar vein, people think "all surges come from a distant outside place and therefore I must fortify the point where power comes into my home".
Well, as Pogo says, "We have met the enemy and he is us!"
Your own appliances are probably responsible for a great deal of the surges you face. So there's a lot of logic to putting surge suppressors closer to the appliances that need protecting. E.G. my first choice of location would be the panel in the residence. Then also any outbuildings with sensitive electronics.
Let's clean up that panel.
First, that deadfront cover. It needs to be installed. The debris is getting in there either because you're running it with the deadfront cover off, or there are additional twist-outs that have been removed. Any empty twist-outs need to either be covered with an approved blank cover (though, they are flimsy) or rivet on steel of the same thickness. I suppose you could tack weld some steel (tack welding is fine; the twist-outs are meant to twist out) but the work needs to be "Neat and Workmanlike" and then you need to paint it. The panel interior paint is weird, it's similar to Glyptal. Glyptal would be fine, actually.
If the deadfront cover is missing, contact Siemens with the model number of the panel and see if they have a deadfront to sell you. Otherwise the panel will need to be replaced. Now there is a "Hail Mary" play that is sometimes possible: Panels have 3 parts: the deadfront cover, the steel box, and that huge plastic thing with everything else mounted on it called the interior. The interior attaches with 2-3 bolts (sometimes one is behind the main breaker). Search the panel for a "Box Number" - on the box or "use with box number ___" sticker on the interior. Then hit Siemens' catalog and see if they still make any "ranch panels" with that same box number. I know Siemens makes a 12-space ranch panel - wouldn't that be nice! If they do, buy it and have your electrician swap interiors next time they visit. The new cover will fit the new interior.
Could you manufacture a deadfront cover? Sure, if it looks professional. Again NEC 110.12 "Neat and workmanlike". The critical dimensions are that it needs to lay reasonably tight against the terraced part of the branch circuit breakers, and catch the outer edge on the left or right side of the breaker. That is the only thing that keeps the breaker from tipping out when you forcefully shut it off.
Next, take a look at that wire going into the 150A breaker. If it is 4/0 aluminum or 2/0 copper, it belongs on the bottom "thru lugs" - the ones currently occupied by that skinny #4 or #2 aluminum. Then the 150A breaker goes away and you have 4 spaces free to a) attach that #4 or #2 aluminum to a 70A or 90A breaker... and b) attach a breaker for the surge.
I can't tell from all the spider condominiums, but it looks like the aluminum cable has strands broken off to fit on those terminals. That is a big, big NO. There is no earthly reason not to put 4/0 aluminum feeder on the bottom lugs. That is specifically what they are for, and yes, they are protected by the 200A breaker up top.
If it is smaller than 4/0 or 2/0, does it go directly into a panel that immediately has an appropriately sized e.g. 150A main breaker? Because I'm not a fan of this, but in that case you can do the same thing, moving it to the "Thru Lugs" and again deleting that 150A breaker to free up 4 breaker spaces. It's not a perfect solution but having 150A wire fused 200A is certainly less bad than having 65-90A wire fused 200A, since 150A wire will endure the overload - 65-90A wire will burn up.
The 4 breaker spaces can be used a) to land that thin #4Al or #2Al wire on a 70A or 90A breaker. And to give 2 breaker spaces dedicated to the surge suppressor. However I'd be more inclined to share the surge onto one of the 60A breakers, and then reserve the last 2 spaces for generator interlock, solar, or whatever comes up.
