I had the same issue, it looks like the custom download folder is specified in the autoupdate_settings.cfg file. I edited it to point to an empty folder and could download the update with no error.
Of course with an alpha there is more to be done, but it's a great step forward. Thanks for all your hard work!
And thanks Wags for laying it out, really appreciate the communication.
I came across this thread on reddit and ordered the glasses from zenni and had the adapter printed, it works great. I just take off my glasses and put on the rift.