I've manufactured such a monstrosity for temporary use, because the outlet on the wall used a 15A outlet (the typical plug you see in the US everywhere) but use

I've manufactured such a monstrosity for temporary use, because the outlet on the wall used a 15A outlet (the typical plug you see in the US everywhere) but used a 20A breaker.There should have been an outlet on the wall that had the 20A outlet (one of the blades is horizontal instead of 2 vertical blades). I used a length of 10/2 solid core wire to make the converter. I took delivery of a APC network-switched power bar that used the 30A twist-lock, and it included it's own pair of 20A breakers. I only plugged 10A (verified using a built in meter) of equipment into it till we could get a proper installation. (I should add this device is not being used in eons and hasn't needed to be used since we've made sure to get the outlets setup right before we get the equipment!)