The hard part is over.
I'm matching you with one of our specialists who will be calling you in the next few minutes.
But if you must use a seat riser, I used to use caulk to form a waterproof seal before switching over to a commode over the toilet.
I looked into replacing the toilet with a 'comfort height' one, but it is an old house and it is unlikely the plumbing would accommodate a new toilet. The drain pipe would have to be moved and that is far too expensive a remodel for a house Dad stays in for a couple months a year.
I wonder.... if the open bottom pail that comes with a commode would fit under the toilet seat and extend below where the toilet and extension meet it might keep urine from splashing there...🤔