The original aft cabin berth set-up was bizarre for anyone but kids.  Before buying our '91 I decided to make it into a permanent double mattress-sized 'guest cabin' as close to the MKII as possible.  See photos attached.  I did try a single-depth air mattress first both by putting it on top of the existing cushions (WAY too shallow in space on top to move around) and after removing all the existing cushions (STILL too shallow).  This experiment led me to the following solution to be able to permanently install bedding AND gain some extra storage height on the aft side of the mattress:

Key to comfort and getting the size and heights right was removing and tossing out (I kept some 'just in case') the upholstery and cutting the underlying cushions down to fit a double-sized, quilted/padded mattress cover (I used a single-thickness queen-size cover and it fit ideally -- the newer deep mattress covers would have left too much bulk underneath). 

1. I took the factory-made filler cushion (the plywood backed insert many second-hand owners are missing -- easy to replace with 1/2" or 3/4" plywood and some foam from Walmart or a craft store per Rich's post above) and removed the upholstery (a bit of a mess as it is stapled and glued).  This sides/top of this insert will never be seen again as it will be coved with the mattress cover and sheets for time eternal as the original set-up was largely useless and the new double mattress will be even larger if little kids need to sleep back there with their heads facing the bow.
2. I removed ALL the upholstery from all the other horizontal cushions (don't alter the long, narrow, starboard-side zippered triangular cover in any way) and measured and cut with a serrated kitchen knife the largest cushion from the starboard side to the port side to remove the aft-most rectangular portion.  The finished cut piece was as wide as the distance from the mirrored wet-locker door to the edge of the shelf behind the companionway stairs -- about the width of a double mattress. 
3. The space aft of the new mattress athwartship now is free of cushion from the fiberglass undershelf up to the sole of the cockpit above and is used to store the lifejacket bags and bike helmets/baskets.  These can be removed when larger guests are using the cabin or guests need some space back their to put their bags.
4. The remaining small rectangular cushion that was used to fill the space beside the wet-locker I cut to fit the small square space aft of the insert, matching the width of the double mattress.
5. I cut the thin triangular cushion on the starboard side to the same width as the new double mattress width and replace the smaller rectangular piece back into the zippered bag and zipped it in, then put the mostly empty bag back in place at the forward end of the space, tucking the upholstered piece under the seat-back cushion and smoothing the empty portion of the bag down, then placed the cut piece back in place on top of it.
6. I threw away the pieces that used to fill the aft-most portion of the space along with most of the old upholstery.
7. Fit the double (or queen) sized quilted, elastic-sided mattress cover over the exposed and fitted-together raw cushion pieces, add your fitted sheets and top sheets/blankets and you now have a semi-Mark II permanent aft guest cabin that your guests can sit up in bed and read in, plus have a small space to pull on their pants in behind the folding doors with storage space for their bags under the insert and/or to the aft of the mattress. 
8. Note that, while the insert remains a bit of a poor fit (as you put weight on the aft side, the forward side lifts and creaks a bit), adding a thin piece of wood on the starboard edge under the plywood on top of the wooden strip and gluing it in place brought up that side to the correct level, and using a table saw to make it thinner at the aft end more or less fixed the poor fit.

Makes a big difference for having people stay over on a Mk I as the bed is always ready to use.  I cover the blankets with a 3' x 4' rubber trunk-liner to keep it clean when I use the space for temporary storage (project materials and tools) between entertaining guests.  :-)