Building or buying an IVR / step down voltage buck converter for the instruments

If you're looking at $60 to get the parts to build a linear regulator type limiter, just spring for the ready made solid state one from RTE. Better response than linear units. Why build when you can get a better, proven unit ready to go for the same money?
RTE limiter - rte
The issue with cheap units (although they do work) is the response time- they take a while to warm up and begin to register properly.