Jerry, I had a metal mk2a left hand side panel altered by the chap who painted my bike who is also a panel beater (I believe?). So if you can get one of these then it can be done.
This panel already had a hole on the top edge, but it was slightly further forwards than my other MK3 panel. I don't know if this is fact, or my MK2a panel was flawed? I use a piece of flat s/steel bar bolted to the front gusset left hand hole (as I don't have the o/e plastic air filter on), and with a threaded hole in the flat bar for a bolt that locates the panel on the top fixing. In the panel hole I fit a rubber grommet which stops the panel being scratched and helps secure the bolt from unscrewing.
Where the hole was at the bottom right I think he welded some bar in and finished it off so from the outside that this has been done, you wouldn't know there was ever a hole there and is very tidy on the inside.
At the front edge he welded some plate on to replicate it to the MK3 shape, and again you cannot tell from the outside and it is a very tidy job on the inside.
At the bottom, he made a tab (similar to the right hand side tab) with a hole in it and welded this on.
You cannot tell outwardly that this is not an original MK3 l/h panel.
If you want, I can send pictures tomorrow?
All you need then is a MK2a side panel.
Added at 21.33 bst. I have read a post by LAB, and if I understand correctly, the 850 mk1a and 850 mk 2a with the plastic airbox will be as the one I have modified but not the 850 mk1 or 850 mk2.
mk1a-mk2-plastic-aairbox-t18926.html?hilit=mk 2a side panel