You can literally just cut the neutral prong off the 14-50 adapter, then it will work in 3 outlet types; 14-30, 14-50 and 14-60. The neutral is not used in the UMC, and it's just a dummy pin. Just be sure to set your max current to 24A when using it on a 14-30 or you will trip the breaker.
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I grabbed the pin with vise grip pliers and ripped it out, then just filled the hole with sealant. I use it all the time and it works perfectly.