After much debate, I decided on a GIVI Outback Trekker 42l aluminum topcase and mounting hardware instead of going with the plastic vario top case.
I figured I prefer the look and durability of the aluminum
Price-wise they were damn near close
I've heard reports of the vario mechanism getting sluggish
My original bmw aluminum rack had an ever so slight bend in it at the wing tips that prevented the vario case from mounting.
So I got this in the 42 litre version. I think the 42 looks just right and not a 'trunk' on the back. I chose silver over black so it doesn't heat up in the sun (maybe inconsequential?)
The product codes: Givi Outback Trekker 42l topcase (GIOBKN42A) with Givi mounting plate (SRA5127).
I mounted it to my F850GS which had the original BMW aluminum passenger grabrails.
The mounting hardware is bolted into the rearest vario insert and one of the bolt points for the grabrail.
This puts the top plates/braces above the original BMW aluminum plate and plastic cover.
There are spacers/inserts which lift the plates/braces above the original BMW plastic cover. Note the right and left braces/plates are mounted to the support brackets to a forward point and rear point** (as seen on the right side of the photo). The three chamfered holes are screw points that attach the mounting plates/brackets to the BMW aluminum plate and plastic cover.
After attaching the actual GIVI top plate (silver) there was a significant bend in the plate which didn't allow the top case to mount properly (couldn't get it to click)...also I have no idea how to rotate pics and the automatic uploader tool constantly kept rotating my photo when uploading.
To solve this bend I placed 2x 2-3mm washers under the forward mounting point for the top plates/braces (between the black support bracket and top black plate) which straightened the GIVI silver mounting plate out**(see stars above).
I went on to try to attach the backrests but...
The proper backrest with cutouts for the strap mounting points should be OBKN42 [V.2018] (code GIVI E173)
The incorrect backrest with no cutouts are for the older version of this case (code GIVI E157).
Also, the Kappa mesh E168K fits the case and is not the same mesh diameters as for the larger versions of the Outback Trekker Case.
The opening/locking mechanism/latch is quite secure, easy to manipulate with gloves for both opening the lid and removing the topcase. The key opens both the case top and the locking mechanism to the GIVI plate. The key can only be taken out in the 'locked' position. You can 'lock' the mechanism without the latch actually holding the lid. When locked, the latch cannot attach to the lid to lock it. You have to open the mechanism, engage the latch, then lock it.
It came with a foam 'floor', which was a nice touch to even out the bottom.