I think if it were me I would make the ladder out of ABS pipe. "T"s' for the junctions and straight pipe for everything else. Use an end cap on all four ends to protect the floor and little fingers at the top. "C" clamps can be used to bolt it to the bed. Clean your cut ends well and use lots of glue. Keep your spans short and no over weight adults!
"We were not placed on this earth to see through one another, but rather to see one another through!"
2013 Timber Ridge, 2013 GMC Sierra 6.2 L