I bought two ski rack attachments on Fleabay. They initially had rubber straps made for them but those dried to the point where I didn't trust them anymore so now I use beefy bungee cords.
Then I built a pretty basic rack out of 2x2 lumber and some wood screws. (I wish I had welding tools to do this in metal instead but I had to go the KLR route.) Then I rattle-canned it black.
The holes in it line up with the slots in the factory rack where I mount it with wingnuts for easy-on/easy-off.
Since those German numbnuts at BMW made the factory rack out of plastic it does move around a bit but not too badly. I initially built it for my K1100LT but my K1100RS has a metal Reynolds Rack so I'm going to try that one of these days.