I did a conversion on a Holophane Bantam 2000 to 39w CMH at one point. I made a small bracket to adapt the existing medium base lampholder mounting holes to a G12 lampholder. As for the height, I made it so it roughly matched the arc tube on the 175w MH it was originally designed for.
Note that there also exist E26 to G21 screw in adapters. However the plastic on mine wasn't suitable for the heat of a 150w CMH lamp