I bought this stuff called Nu-Foam from a fabric store over regular foam.  I thought it would do the job.  I ended up using some of it, along with one of those things you kneel on in the garden (don't ask!).  See down below...

I also bought marine waterproof vinyl for the cover.

This is the material I used.  Pink stuff is used for firmness, Nu-Foam for comfort padding.

I duct taped everything to hold in place while I covered.

I know, it's damn ugly.

Of course, you take the old cover off. 

Make sure you have enough slack in the back.  My first attempt I came up 3 inches short at the end when I got there.  Had to start all over again.

I started stapling in the front and trimmed as I went.

Finished staple job.  Getting the cover tight was tricky.  I didn't pull too hard because I figured I'd rip the fabric or something.  It's tight enough for me.

Here it is before it went on

On the bike...

On the bike...

From behind.