I think all of the suggestions here are valid in cases of “normal diarrhea” but radiation really changes the ballgame. Even my own suggestion for Kaopectate is probably not gonna work.
I think something stronger to reduce intestinal motility is what is needed - an RX for something like Lomotil. All the probiotics are going to do is produce more gas and yogurt, kefir, etc were expressly forbidden on my diet from Sloan as they contain live cultures which normally is a GOOD thing but not during this treatment.
Talk to your RO about getting a prescription for something stronger than OTC remedies; you’re not the first person with this issue!

I added Gasex which is working. RO suggested Metamucil and start with 1/4 Imodium and titrate up from there. I have been using Chia seeds as they absorb water well. RO's solution worked overnight until it didn't this morning. Looks like every 12 hours to start with. The awful thing about diarrhea is when you feel empty there can be left over stool in the rectum due to the bowels still working which gets one thrown off the island. 12 sessions to go and feeling wretched. "Onward through the fog"