Installing an overhead additional bed?

[Mark-camper]

we're looking at buying an VW Autosleeper Topaz, or the VW Autosleeper Trophy, but we need more than one double bed. I've seen some rare examples which incorporate sliding boards above the double bed where an additional person can sleep. Has anyone added this second bed platform themselves? I'm interested to know if its possible to retro fit this into a former double bed version, and what the things to be wary of might be? Many thanks in advance.