This quite common in Canada in the summer months. Most do it via a net slung underneath.
The "keep it simple" is usually the best way, but I saw pictures from NZ of some rack that was strapped on top of the skids on AS-350 and thought maybe someone was making approved racks but then again that was probably just some home-made rack and that will surely not work in the lovely world of EASA regulations so I think underslung is the only "right" way to do it.
SWAGMAN Bike Racks
this could be the solution to do it and sling the whole thing