Hi tonnilerche
I haven't seen anything commercial to help with this need. Being curious myself, I made an adaptation of a confectioner's icing bag out of really dense fabric (pillow ticking) and attached it to one of my homemade inflatable nozzles. These have an internal diameter of about 20 mm. I used a thick-ish mealie meal (corn, US) porridge. Filling the bag was messy in the extreme, and it then had to be sealed by folding the top over and twisting.
It barely worked. Fortunately I had made the bag very large, and didn't fill it enirely. To make anything go up my bum at all I had to twist the bag a short way from the nozzle, so that only a fist-sized lump of porridge was in the now-cut-off part adjacent to the nozzle. If I squeezed this very hard with my free hand, I could force some goo in!
Learning: it takes A LOT of force to get it in, even through a wide nozzle. The bag should have a wide enough mouth to get the small-ish porridge saucepan right into it before scraping the porridge out! Sealing the bag's free top end needs two wooden jaws and a good strong carpenter's clamp.
If I were to do this again, I would make a delivery system with a bag reservoir and a straight section like a firehose about 300 mm long (a foot) between the bag and the nozzle. The firehose bit would be about 75 mm (say, 3") diameter. To get it to work, I would fill the bag with porridge, squeeze it into the firehose. Then sitting on the floor, with the bag and hose in front of me, I's use something like a bottle or an old fashioned rolling pin to roll the firehose flat from the remote end towards my eagerly waiting rectum, already plugged with the inflatable nozzle.
But if you make the porridge stiff enough to be formed into artificial turds you can use an insertable pump directly into the anus. This subject is covered in my posts elsewhere on this site, and is now my preferred practice.
Johnny