Ive installed my coilovers a bit differently then most. Im using Apexi Mazda Miata coilovers with the original s30 top hats. I was having a hard time trying to find steel adapter sleeves with the right thread pitch as the Apexi coilover body, so I just used the aluminum sleeves the coilovers came with.
Basically, cut off bottom portion of the aluminum sleeve so its just a tube. Then cut the strut housing down so the coilover sleeve can fit over. Then drill two holes through the sleeve and strut housing. Then use locking bolts and nuts to bring everything together. So far in two years I haven't had any problems. But nevertheless my futures plans are to find or machine some adapter sleeves and just weld it on just as h4nsm0l3m4n suggests.