Hi Ross,
Thanks for the input! I didn't think about the bowing from welding, I thought 15mm would be overkill but now feel better about it
That's a very good idea with the spacers + bolts, I was thinking in the future if I ever needed to change the top plate for a different mount then it would be easier to go for the rat cage approach.
Can you post a pic of it's layout? I'd probably go atleast 70mm as you mentioned, probably 100mm