You should be able to set "U isotope selection" to 235 only, but I can see now that the DRS will still insist on 238 being there. I'll fix that for the next release.
In the meantime, you were on the right track I think. You should be able to create a U238 channel using the calculator. The trick is that the DRS does not look for "U238" it looks for a channel with its "Mass" property set to ~ 238 so that machines recording as "m238", for example, still work. After you create the channel with the calculator (being sure to create it as an input channel), open up the python console via the Tools menu and enter:
I think that should do it, but please do let us know if not!
All the best,