I found a
lightweight, three-step, ladder on eBay that has already proven itself for me. I do not like flying without looking in my fuel tanks nor guessing if I'm going to spill a quart of oil on top of the rear engine. It's rated for 330#, mine weighs less than 5 pounds and fits easily across the back of the aft bulkhead. I lay it flat on the floor and place a storage box and bags between the legs and rungs. No wasted space.
http://ranchdogoutdoors.com/pilot/33.../ladder_01.jpg
http://ranchdogoutdoors.com/pilot/33.../ladder_02.jpg