none that i am aware of. wrp used to make a few but went of of the trailing arm style seats.
if your seat cover is in good shape, your local upholstery shop can rework your seat and raise it up while still using your stock seat cover. usually inexpensive, guessing under $250. you can add foam up to the top of the hump above the "trunk" and still use the cover.
if you opt for this route, use a hole saw on the underside of the seat's plastic frame and drill holes inside every square. lightens it up a bit and lets the water out.
unrelated to your seat question, 4000 is normal engagement for the mtn max. you can play with different springs and bring it down a little to reduce the "yamaha hump" left behind every time you squeeze the trigger.