PVC Air Horn Plans [Complete Step By Step Details] The plans you will receive are for a 3” diaphragm train horn, but these plans will also work with any size horn you want to build, all you have to do is scale the plans up a bit. Right after you submit your order, a download link will be sent to your email so you will instantly have your PVC Train Horn Guide! PVC Homemade Help Tuning - Train Horn Forums Mar 22, 2012It also talks about, in the plans, using aluminum flashing, which is very thin and didn't work well as a diaphragm. I ended up using 22 gauge steel, " thick. This makes a good sound. For dimension I ended up drawing out what the inside a Nathan air horn is and then scaling it down to the size PVC I am using. I'm using 4" PVC caps.