Dunno whether you or anyone else is interested but I have a spare KR5108 plate to suit a GSA which goes with a KS410 trolley base plate suitable for a Givi top box. I tried to make it work with a BMW softbag but couldnt in the end so its available to anyone who wants to make an offer
The supplier of the Givi Trekker Outback 42 with baseplate told me it was 6 kg of contents of the case, so weight limit is not including the weight of the case itself.
Fair enough but that's not what the manufacturer (Givi) told me - hence why I went with the standard one which was limited to the max capacity of the case, i.e. 10kg