It seems like most Black Triangles suffer a broken post somewhere along the way, and for some reason the left side is the first side to get damaged, probably due in part to the lift handle on that side.
My '96 did not have the extra post that snapped into the side tupperware; that post came in later years. It turned out that the '99 I'm parting out had one good side and the other side had lost the post. Using epoxy to reset the broken post doesn't work as those who tried it found out. There was a repair procedure over on BMW MOA, but that one didn't impress me as the best solution. I decided to try and come up with my own procedure.
I had the good side to study to see what the end result needed to look like.
The broken side:
The solution was found as I looked through the very extensive fastener section at my local Ace store:
The nylon fasteners are 1/4 x 20. The spacer is 1/4" thick and 1/2" diameter. The screw was 1 and 3/4" total or 1 and 1/2" of threads.
A little JB KwikWeld will secure the spacer to the screw.
I made a few of these today and found it was much easier to secure the rotary tool than the screw. I used a drum sander bit for the removal and the stone bit for polishing.
The last three pictures are of the same post though the don't look like it. The unexpanded diameter of the grommet on the tupperware is 8mm or 0.320 inch. I ground the head of the post down to 10mm or 0.390 inch on one axis and 8mm or 0.320 inch on the perpendicular axis when viewed from the top of the post. The difference mirrors the original design and allows the post to more easily slide through the grommet. There will be a bit of of grinding and test fitting until you have it right. You'll note the bottom of the head is also rounded to help the post pop back out when you go to remove it. The head measures 9mm or 0.360 from its top to bottom.
Use a small drill to ensure you are in the center of the the old post.
Enlarge that hole to 1/4".
Cut the post to a total length of approximately 36mm or 1.420 inch.
Position the post so that the top of the head is 18mm or 0.720 inch above the flat base mounting area. You can get under the mounting surface to help get the post at the correct height. Use some epoxy on the top side to secure it in place.
Once the epoxy sets on the top add a generous amount to the underside of the mounting surface.
The repair seems to be very strong, and the dollar cost is minimal, certainly a better option than forking out $80 for a new one.
I'll move this down to the DIY section after it spends a little time here. Edit: In the original version of this post I suggested that you might want to fill in the threads with epoxy for added strength. I ran some tests today to gauge the strength of the nylon screw in its natural condition. You definitely do not need to do anything to improve the strength of the screw. It is very resistant to breaking even when bent at angles that would not be possible when removing your triangle.