Can anyone recommend a cushion to relive pressure on bum/hips?

[Cocolapew]

I was looking at the gel type ones on ebay and wondered if they are any good or comfortable? Or if I should look at memory foam?
It's for my Dad who has lost a lot of weight and also has bone cancer and tumours on his lower back and pelvis.
He's sitting on his chair at home and he's struggling to get comfy.

It was this type of thing

This isn't a cushion, but, I can highly recommend a leg pillow I bought 6 weeks ago. It's called Aidapt Foam Leg Raiser and it's best used in bed, or, on the floor. It provides pain relief for problems with back, hips, knees, and, legs. It's really comfortable whether you lay on your back or side.
It's not memory foam, it's made of high density foam. Very soft washable case comes with it
Price is £25.99 or £21.66 with vat relief from Superdrug. You order it online and they deliver it (I haven't seen one of these in their stores). I've just seen one on Ebay. Have a look there if possible as the photos are a little better on there x

There's JML Egg Sitter Support Cushion too (for when he's sat in a chair, or, on the sofa)

I have a chunky sacral cushion that my son got for me from amazon.
IME memory foam is too squishy and gel cushions too thin.
I find a high density, 4 to 6 inches thick cushion better.
You might get some recommendations from MacMillan nurses, or from your local hospice.

Thank you, I'll have a look at these