I doubt you're going to find instructions on how to build an MVS->AES converter. Those who do know how to make them can get quite a lot for them, so it's not in their best interests to tell you how to make one yourself.
Plus, it may be pretty complicated to make one (not using off-the-shelf parts) - I don't know.
I would get !Arcade!'s converter once it's available or just get an MVS.
Well, the program board is only a matter of wiring, but graphics board is not as easy to convert. As far as I know the problem is MVS carts outputs 2 data buses as AES carts only output one. So you need to add some mysterious components to make the converter select the right bus at the right time (which is done within AES carts for home systems).