It is very easy to make an SIO2PC-USB device using an FT232R chip from FTDI. Mine was based on a breakout board for that chip which in turn was made by 'Sparkfun'. However. while participating in another thread today it was brought to my notice there are actually several different devices that do the same job which also work with RespeQt.
Previously I had believed all the commercial devices were just variations on implementation of the FTDI IC. Apparently this is not the case and the AtariMax SIO2PC at least uses a custom programmed microcontroller to more quickly handle communication from the Atari to the emulated drives it mimicks. Are there any others? More to the point, are there any other SIO2PC designs that are open and possible to build for oneself?