I doubt that. An engineering approach: at the end of the day it's a simple turned piece with a hole drilled through it but with a 1/2in A/F hex machined on one end. The catch is that it will probably have to be heat treated to harden it meaning that you aren't going to be able to get a copy made for much less, if any less than the Makita original, unless you change the design, which you didn't mention doing. Looking around the Makita is a relatively new product and differs from all the other earth auger motors on the market (petrol, corded electric and battery) by having a chuck instead of a direct adaptor, ergo the reason you can't get an equivalent is because the Mak is currently the only game in town and it's hardly a big market
To my mind it would be lot cheaper to take a piece of something like, say EN39 bar stock of the right diameter, drill a hole through the side to take the pin, drill a hole in the other end (I believe 29/64in from my Zeus) and cut an internal thread of 1/2in x 20tpi UNF so that you can thread this device onto a standard drill, once the chuck is removed. Should cost you a bit less than the Mak item if you know someone with a lathe (under an hour's work) and if you could get a piece of appropriate diameter bar stock is a job you could do yourself with a drill press, drill bits and a 1/2in x 20 UNF tap. It means that the (home-made) adaptor could be used instead of a chuck - but I still have my doubts that without considerable modification it would be safe on any drill, the issue to me being the ability to hold the speed constant at low revs. But I'm sure you thought of all this