Richard, as earlier post,I use exactly that - P5100, UR-E20, Kowa 773 (with zoom eyepiece), Kowa DA10 which screws on the eyepiece instead of the eyecup - and the Kowa 28mm adapter. The adapter is basically what they call a step-up (or -down, depending which way you look!) between the DA10 and the UR-E20. Unfortunately there appears to be no 3rd party replacement for the 28mm Kowa adapter- the Kowa thread is a proprietary size - or 52mm? - or at least I've not found one - which is sad as the Kowa one costs c.£40 now in the UK - so probably $40 your side of the pond.
The UR-E20 is not 52mm, it is 28 mm so you need the Kowa 28mm (in the same way as you use the 37mm on your Sony set-up now )as I say which steps to to whatever the DA10 thread is. I usually keep the 5100, UR-E20, Kowa 28mm adapter and outside (camera side) of the DA10 asemmbled for quick action if I intend to digiscope, with the eyecup replacement side of the DA10 on the scope - then it bayonets on perfectly aligned . . . .
There is the SRB-Griturn replacement for the UR-E20 but its still 28mm!
Good luck, Mike