This is such a common problem, its almost universal that the side-panels on most motorcycles are attached via a few a tulip lugs that push through rubber grommets. Which is great, except that heavy handed removal, or age making the plastic brittle easily sees the lugs snap off.
If you are lucky, the lug stays in the rubber grommet & you don't loose it or the side panel, & can weld or glue the two bits back together, unfortunately, often the case that the jug falls out & gets lost. What do you do?
Well some people get a new or used side-panel & simply replace the whole part. Some, get another broken panel & snap the lug needed off that to weld or glue in place. Others find another way to attach the side panel, drilling holes & using cable ties or wire. Some glue bolts into place where the lug was, or any number of other more or less ingenious, or tidy repair!
But, THIS method, is quite useful. Materials used are virtually free, essentially old scrap plastic; it remakes a pretty good facsimile of the original lug, and fastens it in place so it works exactly like the original, & if you are careful, wont distort or discolour the face side of the panel, so you needn't re-paint or anything.
The new lug will be constructed from welding together strips of old plastic into a 'stick' that's then shaped to suit. I find the sort of plastic from old stak-a-boxes or tup-a-wares most suitable, as its relatively uniform in section & a helpful thickness.
Start buy cutting or snapping a few strips about the right width & quite a bit longer than you need, then welding two strips together to form a T-Section.
Next, weld another strip on the back of the first, to turn the T-section, into an X-Section. Don't worry about making the strips the same width, because once you have the basic stick, the edges can be melted down to size, flowing the excess plastic from the edge into the joint to increase the blend between the two strips.
Once all four sides have been shaped to a uniform thickness and neatened off, the 'head' of the lug that will form the tulip can be roughly shaped the same way, dragging excess plastic from the end down, to build up the gaps in the X to make an almost solid ball of plastic.
Beneath that head, though a flat base is needed where the lug will seat against the grommet. For that, the sections are melted into a notch, and another strip of plastic welded into place in the notch, the excess again, used to build up the valley and form a flat platform.
Repeated on all four edges, and shaped, the neck of the lug, and the tulip head can be sculpted, if needs be, more plastic added to build to shape.
Once the Lug is formed, allow to cool and set. It can then be trimmed to length & positioned on the side panel, before being plastic welded into place. A trial fit may be needed to accurately align the lug, and it may be that the lug proves too large or small to fit comfortably in the grommet.
Depending on how the attached lug fits, its possible, once attached to the side-panel, to use the soldering iron to do a little more sculpting to to build up or move down the grommet base, or to size / shape the tulip head more comfortably. Final fettling can be done after the plastic has cooled with a hand file for smaller adjustments.