Willyum,
As you can see, I don't visit every thread every day! Missed your question about mouldable material insertions. I have done this with a proprietary mixture of plaster of Paris and boiled cornflour gel.
I had to experiment a lot to find a mix that would not set like a rock, because I had precisely your own reservations about the safety of doing this. I was very successful, but having no butt plug or other device available (this was over 30 years ago), holding the mixture in until it set (ca. 20 minutes) was a MISSION!
I stopped doing it for this reason, though the sensations of carrying the mass around in my rectum were quite delightful! The urgency to expel was quite intense, but the mass was almost three inches in diameter and over eight long, of a consistency solid enough so that if I had had to extract it mechanically, I would have needed a teaspoon to dig into it successfully. Fortunately this never happened, and I was able to push it out OK. But I had inserted enough of the semi-liquid to overflow into the descending colon; and there it stuck, preventing the ingress of an enema by which I had hoped to dislodge it! I needn't have worried: it came out by itself in a few hours.
If I were to do this again today, I would first shave my anal area, inject the load, clean and dry off the perineum, and then apply that wide adhesive sports strapping over my anus. That would help me hold in the very irritating mixture until it had set!
Off topic, but interesting: the adhesive strapping also works to keep hemorrhoids from protruding and causing pain! But be careful of allergic reactions to the adhesive.
Johnny