What gauge (thickness) steel should I use to make the restirctor plate.

 

When I was NonSeq I made one out of 1/8" aluminum plate and drilled a 2" hole in the middle of it. It worked fine. After a year or so with it in I went single turbo and took it out. The plate held up but was bowed toward the back of the car

 

1/16 is more than enough as long as you're using a good (316) steel