There has been various variations of that spacer type made by several people.
The trick with carb spacers is results will vary depending on your application. I've made several for myself, but am reluctant to sell them for that reason.
Something that might make big gains on a 300 ci high RPM engine, may not do much on a 300 HP 360 with a dual plane. They're not a one size fits all kind of deal in my experience.