Yesterday I got a 115-20 centering ring and a short section of 175 tube to add to the 115-175 ring I already cut to see what would work with the quad motor mount to block the forward ends of the 18mm tubes. The 115-20 blocks the quad tubes but is a tight fit between the fin tabs so would take some work. The 175 tube perfectly aligns with the outer edges of the four 18mm motor tubes. So I’ll cut an 1/8” ring and glue that to the 115-175 ring which will make a crude approximation or Semroc’s intricate laser cut part

I’m trying to approximate the middle ring of the quad mount that transitions from four 18 mm tubes to one 29mm tube (Semroc 115 tube). You may be wondering why there is a transition to the 29mm tube at all. There is a half baffle in the rocket design, and the 115 tube should direct the flame into the coupler of the baffle instead of allowing the flames to exit directly out the holes of the baffle. I had already slotted a 115-175 centering ring to accept the parts Semroc had remaining for the quad mount, the forward and aft spacers. Since the 115-175 centering ring doesn’t quite cover the edges of the four 18mm tubes (ST-7s), I got a short section of LT-175, and using the NCR Tube cutter, sliced about an 1/8” ring to go over the 115-175 centering ring. The last picture shows a dry fit of the middle ring in place between the quad 18s and the LT-115. Next step will be to complete the aft ring which is being made out of a CR-115-225, so the quad 18s will need to be cut out of it.

I believe that kit used 3/16" lugs, but nothing wrong with going with 1/4".
I built all the SLS kits but the Laser-X and Sky Hook. All used 3/16" including the Arcas which is H-rated.

A little late to the party, here's a scan of the quad centering rings I purchased from erockets a year or two ago. Also let me know if you need a scan of the decal, I see that one is being sent to you in the mail. I have a full kit I scored at a great price (below MSRP) from eBay a while back and like you decided to build the optional interchangeable mounts. Hope this helps.
Laters.

Thanks, that's really cool because I have never seen those parts scanned or pictures before assembly, only the finished mount. The middle ring is a work of art.

I attached this pic before but you can see where I am with making all the extra slots in the rings for the quad mount, im using a 225 ring set from erockets and a 115-175 ring plus a sliver of 175 tube. I think I'll glue the four ST-7s together and trace around them for the aft ring openings.

It does, I am thinking rail buttons also but the mount is a problem with those.

I would go 1/4" if you are going with the higher end 29mm motors such as G64/G76/G78/G80/G138.

Got the 1/4 inch from erockets, they are cool since they are just like smaller lugs not thick like LOC.

The original part and my attempt to make a workable replacement:

Having this scan of the rings was a godsend because the clover leaf ring that the four ST-7s pass through would be tough to make accurately without the scan. I printed it out which at 100% came out perfectly. I had already used one of the 29mm mount rings to mark and cut the slots for the four cooling vanes. I cut out the pattern and put the vanes through the pattern and into the slots to hold the pattern in alignment and mark the four half circles to be removed. I used a Dremel sanding drum and sanded away the wood just past the marked line. The fit is not perfect but it’s pretty good. Last pic is a dry fit of the mount. With all the interlocking parts it’s a little surprising the top of the ST-7s are simply glued to the narrow ring/115 tube. It will need to be checked after each flight to make sure it is still ok.