Then use a bread knife to cut a series of parallel cuts to the correct depth. Cut these about 1 inch or so apart. This is so you can cut each one out seperately and better control the depth:

 

Remove the smaller foam section - controlling the depth using the serated bread knife:

 

Top done:

 

This is how I did the sides - I left the top full width because I wanted the resultant seat to be a little bit wider than the stock seat which will happen when the gel pad folds over the edge. If you want to maintain the stock profile, trim to taste:

 

If you get blood on the foam, don't worry it will be covered [:@]

 

Test fit:

 

 

 

Use scissors to trim as needed. Use this stuff to glue it into place.

 

This is the memory foam - I've trimmed the edges a bit so that the tuck into the side recess as much as possible.