My longest day on my homemade seat was 650 miles.
I made the seat by removing the stock cover and padding and attaching a $12 15" x 20" Poly Cutting Board from Walmart to the base pan. I left the back half 15" wide and narrowed the front down to stock width at the very front. I put two 1" layers of semi firm foam (firmer than fabric store pillow foam) from the internet on the board and attached it with staples and glue.
It has lasted over 60,000 miles and is being relocated to my newer KLR and recovered to match the color. While cruising I can sit back on the nice wide soft back section and sslide up to the narrower front for more technical work. The padding is about 2" thick. I have a 30" inseam and can flat foot it from the front.