Just put everything on that heavy DIY plate, and it's just a piece of cake for this robust mount.
I cleaned the GOTO Mark-X last night and it looks very fine now.
I've attached a small ball head and a small pan plate to that heavy DIY mounting plate to see how the whole thing works.
Even when the mount is not balance like this, it moves very smoothly. I'm very impressed and really longing to have a motor for it!