Actually I came up with a design for cooling a SCT, it would apply equally to a Mak Newt too. I adapted the idea from Bird who made one for his Newtonian.
See this link for
cooling a C14
I have also seen Mak Newts made up to 10" by
Stellar Optics
The look pretty nice but cost a lot of money. A Mak Cass might be better still and Stellar make them too.