It blows my mind that there's a market for 'manufactured' sheet stock splitters. It seems that if your car is 'engineered' to the point where you're splitting hairs on a splitter
, than your ingenuity and fabrication skills should far exceed the ability to measure and use an Ofla knife.
I'm doubtful TDR or any other miata vendor produces in enough volume to get price breaks on materials - especially on plastics. This is one of the product instances where there's really no monetary advantage to buying it pre-made (a cast turbo manifold being on the opposite end of the spectrum due to the costs of development and manufacturing).
Just buy a piece of ABS or PVC plastic from a local supplier/sign shop for <$30, spend an hour with some cardboard and cut out whatever damn shape you want. Depending on the how thick the material is you can get the job done with a utility knife, some vigorous scribing and a gentle *snap*.
OR ... if your picky you can make a template from scrap plywood or MDF and router it out.
OR ... if your picky and have some resources (and have access to AI, SoldWrks, Pro, Rhino, etc.) then you can make a dwg file and have it water jetted or routered by a local shop. With this method your getting pretty close to the cost of the TDR piece but at least you get the shape, fit and material choice you want.