Tried this. If you USE your MacBook while it's charging, it will actually not sustain its battery strength, it will drain, albeit more slowly. If you leave it alone, it will charge but take SEVERAL hours to complete the charge. This is the only charger on Amazon that produces the proper output (29W is 14.5V at 2A) to charge at the same speed as the original adapter, but it ships from China in a month from now.
Helper

Mini 14.5V 2A 29W Car Charger Car Adapter USB-C for New MacBook Mac Book Retina 12" with 5V 2A USB
https://www.amazon.com/dp/B01E47PWEI/ref=cm_sw_r_cp_tai_i7SuxbH59BVB0
Anything charging over a USB-A connection won't produce or shouldn't allow the power that a straight USB-C cable does, as it's not designed for that. Any manufacturer that tells you so is wrong. And any generic USB-C charger still won't produce the wattage for the MacBook, as they're designed to charge newer USB-C smartphones.
Of course, a quicker solution is to get a cheaper inverter so you can simply use your original adapter as if it's plugging into an AC outlet.