Any sub that you like that is specified for the net volume you have, if they list the enclosure gross volume then you will have to subtract the driver displacement to figure out that net volume. I like Fi, ascendant audio and SSA could be alternatives for SQ subs.
Re Se's sound nice, but idk about putting 1200w to it. Never used an Rl-p but have used other tc-9 based drivers and loved them. Never heard an SSD sealed.
RD Sonance performs great in sealed box. Very very flat response with solid output. Make sure to get flatwound coil if you do look into one. I think the Atomic Quantum is basically the same thing according to Atomic.
What about the ID max? heard alot of rave about those in sealed boxes, but never heard one myself.