Hi Cla, you do have a job on your hands but God bless you for being there.
Two or three things to help, really help!!
1. Eakin Seals. The four-inch seal with a hole in the middle. The material is soft and pliable, it can be molded and shaped to fit and fill any creases. There is a cream version but it's very messy and awkward to use. The seal/washer/pad goes on the belly first and squeeze it down on the inside edges.
2. I use the Hollister One Piece Convex Wafer. Mine has a stiff plastic outside layer which curves into the stoma, Convex Wafer. This pushes the edges of the circular hole and the Eakin Seal right down into the stoma edges. This will stop output from getting under the wafer.
3. Micropore Surgical Tape made by 3M, only this, 3M will work. I use 2-inch tape on the four sides of the wafer. This is very thin tape so the strip along the navel side will stick tight enough to seal the navel area.
4. One more thing you really need is something called "Pearls" (or Absorbagel). It's a salt-like powder that comes in little packets. Every time the bag is emptied, pop one of these into the bag. This will absorb much of the liquid in the bag and the thicker the output is in the bag, the less likely you will have a leak. It works fantastically. The patient will get much better sleep also.
This is what works flawlessly for me, should for any ileostomy. I have a large midline scar and the tape works wonders.
All the best, and stay safe from Covid.
Eamon.