As per slolane's thread, using his inspiration, I came up with this bracket system. I used 1inch square aluminum tubing with steel hex bolts and steel T-braces to mount the bracket to the undertail of the bike.
I think overall, with the hardcase on the bike, it looks pretty good. Once removed, those brackets are ugly as hell. I have yet to paint them because I think I want to make a few tweaks to the height and width of the brackets so that it holds the cases closer to the bike. After I paint them black or maybe, just maybe, wrap them in carbon fiber cloth, it'll hopefully look less out of place.
I went this route because it would allow for passengers with the location of the cases farther back, and because it is easily removable as givi has intended. This also allows me to move the givi cases to another bike if I so choose.
Here is the most comprehensive list I can come up with:
From Homedepot:
*5/16in Hex bolts w/ washers + nylon locknuts -- should have used 1/4in hex bolts
*1in square aluminum tubing -- might try something thinner or round tubing
*1/2in round aluminum tubing -- used as spacers in the undertail mount around the M6 screws for support
*T-brace to connect the square tubing -- I really wish I could weld the joints instead.
From Lowes:
*M6 x 75mm Hex bolts -- Lowes is the most local place I could find M6 bolts in that length. At other locations the max length is 50mm and that makes it slightly too short. I think I could have also gotten away with 65mm length bolts so that it'll be a shorter distance and thus a stronger mount.
From Twisted-Throttle:
*
SW-Motech Quicklock assembly that works with all Givi monokey cases. Just needs to find a way to set them up.
Some caveats of this method:
*without a welder or some way to join pieces of metal it becomes a major pain - I ended up using a hacksaw most of the time just because I could go slow and take my time.
*also because I had to use T-braces and bolts, the whole bracket is heavier than I would like it.
If I were to do it again... I would change the type of tubing I used. Square aluminum tubing is super strong but it looks ugly and doesnt really flow with the style of the bike. I'm thinking about some kind of steel or aluminum tubing and a welder. The bolts also causes it to have a very clunky look. Despite this, I like the location of the cases since it does allow for passengers and makes the bike look a little longer and less compact.
Thanks slolane especially for the inspiration in turning our little ninjas into a more practical bike and possibly a touring bike for me. If anybody has any easy ideas to modify this "prototype" bracket into something sturdier and lighter, please feel free to comment.
Update: To test it's weight limit, I filled the E21's with water and there was little to no sagging anywhere. Looks like those mounts under the tail are pretty strong. I'm impressed.