Passenger handlebars on the mundo are not really stoker bars just some thing for passengers to hang on to. It’s not like they are pedaling or leaning on the bars like a true stoker would be. So I don’t think the position and height is too important.
This may seem like an easy thing to do but I spent a long time on the net zooming in on pictures of other installs to get the details of how to do this on the cheap. I couldn’t find anywhere which explained how to do this in detail hence the following.
The principal of stoker bars on the Mundo is easy, attach some handlebars to the seatpost. I considered proper stoker bar stems but they are not easy to source (in aust anyway ) and they are about $60 to $100 not counting the bars. Yuba have a kit with bars and grips for $49US which looks Ok but I wanted a cheaper solution and the one I came up with is about as cheap as it gets.
I used an “ahead” type bike stem which suits handle bars of 31.8mm (same as the Mundo seatpost), and a steerer clamp of 1 1/8 inch; that way I could reverse the stem using the handlebar clamp on the seat and the steerer clamp to hold the handlebars. Most stems come with some degree of “rise”, usually 6 deg or so, and of course if you reverse the stem this rise will turn into a sideways offset. So you need either an adjustable stem which you adjust to 0 deg or just get a 0 deg stem, which is what I used. These are not as easy to find as riser types but they are available.
Ritchey do an adjustable stem for about $20-30 on JensonUSA.com but I just got a very cheap ($6-!) Answer AtacPro 0 deg, 120mm stem (also from Jensen). I also needed a shim for my handlebars, but this depends on what bars you use.
The bars I used were cruiser/riser type (again cheap Dimension brand about $15) but I think they were too wide and I cut them down to avoid taking out pedestrians! I made them slightly narrower than the side loaders.
Total cost all new parts (excepts grips) about $25- and looks like a bought one.