The beauty (and the challenge) is that everything fits so precisely together. But that also means that if one part is misaligned the whole thing might not work right. Kudos to the genius of Carl McLawhorn.
Dry fit everything before permanently gluing. and watch out for glue boogers where they shouldn't be. Looks like you've got everything under control.

This is a weak point, I added epoxy glue fillets where the top of the ST-7s meet this CR. It's withstood many 4xC6-3 launches with no degradation. Once assembled, hold the mount angled down about 30 degrees, drop a bit of 5 minute epoxy in the corner, hold until set, repeat 3 times.

I concur with 1/4" lug, allows for a longer stiffer launch rod.

It really was genius on Carl’s part, a picture of the intricate laser cut parts of the original mount is what drew me to the the rocket in the first place. I didn’t fully understand what the SLS kits were like until this project. They seemed expensive for their body diameter but I didn’t understand the features and how heavy duty they are.

While I wait for some engine blocks to arrive for the quad mount, I decided to do some more work on the dual mount. The only original parts I had for the dual mount were spacers that go between the centering rings. I was able to get non interchangeable dual ST-9-225 rings, and cut the forward interchangeable notches and the slots for the spacer tabs. I had extra cooling vanes so decided to buy enough ST-9 to cut longer tubes to accommodate the vanes. Today I cut the four tab slots for the vanes in the aft ring so now there are eight slots in it. I followed the instructions Chas sent me and was able to get it together. I left off the vanes for painting, but the pictures below show them dry fit and alongside the quad mount. The first picture is the picture of the stock mount, I thought it needed the vanes added since the quad mount has them.

Glutton for punishment - Making a decorative interchangeable 24mm mount
 

I had noticed that erockets had a plane Jane but interchangeable ST-9(24mm)-225 mount that could be used in the SLS Laser-X so I got it. I had gotten an extra fin set so I had extra cooling vanes and again decided to add them for the look. The vanes are spaced to fit on a 29mm tube so I got a 24-29 mm centering ring and the 29 mm tube will slip over the 24 mm tube on the aft end. You might think this is frivolous since I could use an adapter in the 29mm mount, but the 29mm mount has no motor retention and I’m not sure how I’m going to do motor retention so this gives me positive retention for 24s. An odd thing happened as the motor mount came with two forward rings, so I ordered another set from erockets, but the separate ring set came with the motor tube hole laser cut off-center. You will see in the pics I used one of the rings from the original mount but had to cut pieces to fill the notches in the aft ring. I used the main kit mount for slot templates and I traced and cut 16 slots in the aft ring and four in the fwd ring for all the interlocking parts.

Wow, you have a lot of patience. I would have asked Randy for a replacement ring.
What did you use to cut the tiny slots in the plywood?

Randy told me about a month ago that the cutting files for the intricate parts that were discontinued were not imported to his current system and are gone so I am making them from leftover or simpler parts. The bad rings were .50c on clearance. I emailed and asked Randy to see if the others he had on the site had the same problem and he responded that he cut new ones, which was not my intention, and put them on the site, so as always he was great about it. Unfortunately I had a block of time and was done the cutting when he got back to me, I had been waiting a while to do it.

I used an Exacto #11 with a slightly broken tip. Plunge into the four corners and rock the blade until it hits the cutting surface. Then repeat in the opposite direction. I think these are light ply so it’s not bad. The cuts are visible on the opposite side so the cutting can be finished on the other side. If the slot is too big I put a drop of glue on the edges which when dry usually is enough for a friction fit. It was lot of fun to do. I’m debating on finishing the cooling fins off the model, there are 32! now.

I would definitely finish them off the model if you are painting the vanes a different color than the motor tube. May I suggest using double stick tape and laying all 32 pieces on. Fill and paint one side and let dry. Reverse and do the other side. Sand off the paint/filler on the gluing surfaces and you're good to go. Easy peasy.

You must have a ton of patience.
I would never go to the trouble of making all those cuts by hand with a #11 blade.
If the laser cut part was no longer available from Semroc, it would be omitted....

It was fun to do and the plywood was perfect, soft enough to cut and not split. It was way easier than it looks. When I did the slots for the dual mount I realized the rest would be doable given enough time, and very rewarding when finished. The double stick tape for finishing the vanes is a great idea. I’m thinking the cooling vanes might be good silver with black edges, white or red tubes.