grab the part number from Happ and ask them. It can't be much.. an entire Super is like $8 new.
And as for the DIY... like I said, go for it. But in this instance I think it is silly and will end up with an inferior product. You're looking to convert an 8way to a 4way... that is not the way to go in my opinion. Knievel says it works fine, and I'm sure a file would overemphasize the cardinal directions, but the fact is you can still hit diagonals. That's not a great 4way then is it? And there's no way to "take out" the diagonals in a restrictor plate that is already an 8way... not without designing an entirely new plate (but again... why? the whole thing is $8)
And.. if you wanted to take a 4 way and convert it to an 8way.. ok that makes more sense in my mind. However, in that instance you'd have a reunion stick and still need to buy a Super handle to match... so again, why bother?
Don't let me talk you out of it.. I've ton tons of things "just to do it" but this seems very silly to me, considering time and cost.