I finally had the chance to try this out on our trip to Jokkmokk in 2014. Here you can see it with our entire travel baggage on it. Somwhere in the region of 100kg. Our working load was a lot smaller than that but it handled this load perfectly.
My only concerns in the end was trying to roll it up for the flight in low temperatures and the weight of it, 7kg took up a lot of our flight baggage allowance.
Another group from BcUK organised a trip to Finland a year or so later and they used 3mm HDPE to build their toboggans instead, which worked well enough but was cheaper and weighed in at much less.
They also had a spare sheet which I bought off them. It has taken me ta while to get around to finally fitting it out, prompted by the prospect of joining them next year on a trip to Kittila.
The way I rigged the heavy sled worked well enough, so I decided to copy that for the new one. The same three oak bars, One to act as an anchor for the hauling straps and the others mainly to discourage the sheet from curling up under the tension of the cargo straps.
The bars are attached to the sheet with 4mm countersunk machine screws running through the sheet, into recessed holes in the bars that accommodate washers and Nylock nuts to prevent them unscrewing in use. I used marine grade stainless steel again which was probably overkill but I prefer to make things right and make them once.