Variac construction suggestion

Hello everyone,
It seems a pretty stupid question to ask, Have anyone ever tried to build a toroidal variac by their own ?
Here collecting variac is very difficult and the price is so high, and most of them are of very big and high powered ,for industrial use. That's why I am thinking about a project to build a variac by myself. Will it be impossible. I can collect smaller toroidal laminated core. But I need technical information about Winding,which is the most difficult part. I want to build a variac that can supply 0-300 V , Is there any resources ,or books on Variac construction, cause it seems the winding is very delicate and the volt/turn calculation as well as core calculation have to be precise. Can you guys give any suggestion, or resources about Variac winding process.?

Thank you