I also recently did similar fix as steve, for my FX 450... my bad knees wont let me stand and pound the trails like i used to... so i bought a GUTS foam, soft, stock height, and took original seat cover off (pryed staples out carefully), removed stock foam carefully (in case i want to use again), and took seat base and new foam to local upholsterer... he charged me $40 bucks to glue new GUTS foam on, and re-staple stock cover over foam... after first ride, realized he used wrong staples, 3/8" instead of 1/4" that came stock (i slid to rear of seat, and got poked by them!)... I did some trial riding, then rode in a 2 hr harescramble... the GUTS foam was tooo soft! i wound up riding right on the plastic base after 30 mins. of being on the seat... no good... so I went home and figured i would do some experimenting.. I went to local paint and upholstery supply shop, and bought 3M adhesive in spray can ($15), went to Harbor Freight and bought air-powered staple gun ($28), went to ACE hardware, and bought 1/2" wide, 1/4" long staples to fit gun ($4), went to Walmart and bought Hamilton Beach electric carving knife ($18), and went home and got to work.. took cover off again, got black sharpie out, and proceeded to cut a 1" thick slice out of GUTS foam... removed rest and set it aside... spent an hour cleaning glue off plastic base, then squirted some 3M on base, and on stock foam, waited 5 mins to get it tacky, and put them together... after an hour or so, took sharpie and drew pattern on stock foam.. removed 5/8" in low part of saddle, and tapered to 0 at front and rear of seat... applied some 3M adhesive to stock foam, and to GUTS 1" sliver, and stuck together... put some weight on it, let is sit for awhile to adhere. Got an air-powered 4" disc sander (also available from Harbor), and proceeded to gently shape 1" addition, to make it closely match the shape of original seat, but leaving cross section slightly wider at top, to give my big butt more room to plant... got it to decent shape, sprayed some 3M on bottom outer edge of seat base, and began to work seat cover on from front first.... put a few staples in, and began stretching it, checking it for wrinkles, and pulling it around base, adding some staples now and then to keep it in place... at the end, it came out looking pretty good for home job, and I can now help friends do it as well!
Took it for a ride, spent a couple of hours on it, and the seat is amazing! stock foam is stiff, and prevents feeling plastic base, and GUTS foam is ultra soft, and very comfortable... did not get HusaButt on long trip!