The seat on my Himalayan is the one thing I could not put up with. So I bought some foam. I can recommend this company who sent me some samples. I chose this fairly high density stuff ...
At first I was going to order a single block, 9cm deep and trim it. Then I realised that building the seat up was more sensible, so I ordered 3 slices each 3cm thick..
Then I set to with a bread knife and some aerosol contact glue from Screwfix.
After an hour ish in the open air and (for a change) sunshine, I ended up with ...
I had a sheet of memory foam which I stuck on top and is currently curing. I aim to cut it slightly bigger than the seat "bum print" so that the vinyl, when stapled to the base, gives a bit of a cushion to the back of the seat. (I intend leaving the pillion seat off and replacing it with a flat surface to strap a bag to.)
Next step is cutting the vinyl to size and stapling it to the base. I'm not going to bother with seams etc, the aim is comfort not style. I've retained the OEM seat cover and foam for any future owner who wants to return it to standard.
It already has them. The reason I needed to change was my height/leg length . I think I'm a bit taller than the target demographic for the bike. I was sitting on the ridge at the back of the seat.