I have seen contractors double-lug the 200 amp disconnect, mixing the 4/0 aluminum conductors with the little tinned copper 12s or 14s from the surge protector in the same lug. Not code since the lugs aren't rated for that and you are mixing copper and aluminum under the same screw. I didn't approve it, and I assume this is what you are trying to avoid.
You "could" use some kind of Polaris tap on the big wires to connect the little wires from the surge protector to the big wires, if you can find one that is rated for both sizes of wire, or if you can gradually step the wire sizes up with several connections until it works. Some surge protection manufacturers specify that they must be terminated on a breaker, but they don't pull any load and really would kind of act as their own fuse, so it's not scientifically necessary. Ugly, but safe and serviceable.
What I would do is wire nut the surge protector to an existing branch circuit in the 30/30 panel, and mount the unit outside of that panel. If you can't put it outside the panel, cram the surge protector unit inside the panel, and install a $2 sight glass like a furnace would have into the dead front so that the little green light on the surge protector is visible when you open the panel door. Again, ugly, but safe and serviceable.
