What are the alternatives to adult diapers?

Colostomy bags.

They have thicker underpants called Depends that are not as obnoxious as diapers.

These may be suitable…

I’m not sure which end is incontinent or if the elderly person is male or female.
After my surgery I had experience with urinary incontinence.
One of the devices i used was an incontinence clamp. Yep . It does what you think it would do.
You clamp it on for a couple of hours to stay dry and, when ready, release the clamp and out it comes. You can do this in a public restroom and nobody is the wiser.

I eventually had sling surgery and that fixed it. Whew!

[Mod says] This question is in General, tempting as it may be.

Is the issue with urine or faeces?

If its urine consider trying drinking more water. The number one cause of urinary incontinence is from dehydration. It sounds counter intuitive but you are dehydrated the urine you produce is son concentrated it irritates the bladder and you physically can’t hold it in.