SEDNA - PCI Express USB 3.1 Gen I ( 5Gbps ) 4 Port Adapter with Low Profile Bracket - Supports .... Mac Pro ( 2008 to 2012 Late Version for $29.90 at amazon
Seems to work fine on my 2008 MAC Pro 3.1
Difficult part was to fit it into the frame(~ 1 minute) rest was automatic. No software drivers etc to install, fully recognized by MAC - El Capitane
$30 is on the high side, but I wanted to be able to return it if it does not work .
I added this as I have problems with the stability of my USB connections. The RFX and the INSTEON modem fail to stay connected. So far so good.
Karl