making "coilovers"..

The best way to adapt coil sleeves onto circlip dampers is to put the coil sleeves onto the circlip, and turn out a step to encapsulate the circlip. For Bilsteins, the ID of an ebay sleeve is a perfect fit around the OD of the damper. For Koni, IIRC the OD of the damper is smaller so I guess it depends on what sleeves you bought on whether or not that will work. Ground control makes a sleeve for Koni, and I believe even Koni makes a cheaper sleeve for Koni. I believe Koni sleeves fit as is, and the ground control uses an adapter ring.

Once you decide how everything fits, I would highly highly recommend putting the sleeves on a lathe and counterboring the ID at the bottom so that the sleeve not only sits on the ring, but encapsulates the ring. I've seen that ring start to buckle out when people just plop something on top of the ring.

Assuming you don't have those resources... and I highly highly recommend going the route of properly fitting the sleeve and turnout out a step in the ID... I plopped sleeves onto the perch when I was in college and it seems OK for short term use. I stacked some thick orings around the shock body to keep the sleeve concentric and stable, and then slid it down onto the factory spring perch. Do not cut down the OD of the spring perch, the outer ring provides a lot of rigidity to prevent the perch from mushrooming out. I think I recall the circlip starting to do some sketchy things though at which point I made some encapsulating adapters in the school lathe.

Again, I highly recommend figuring out how to adapter or machine the sleeves to encapsulate the ring... I've done a few iterations of short cuts and every time the circlip started trying to escape. I've never dropped a sleeve, but if it sketched me out in college when I was broke and had zero self-preservation it must have been pretty sketchy.

Merkurs do this as well to convert the stock struts to "coilovers".

The "ground control" store that sells those kits might have something that works (or at least ideas) for you as well:
https://groundcontrolstore.com/collecti … azda-miata

You know, it had never occurred to me there would be an X1/9 enthusiast forum, but here we are.