I roughed up the plastic with some 60 grit sandpaper & a course file (for the nooks & crannies). Then I glassed it with an epoxy resin & cloth mat. I decided to do the entire inside of the cover to protect against further cracks & breaks. It's held up really really well. It is heavier than a stock side cover, but with the price & scarcity of new/used ones, I'm willing to add an extra few ounces to the total weight of my bike.
Also, for the front holes & cracks, you can mix super glue with baking soda (as a filler). Then sand smooth & repaint. That works better for smaller cracks & pinholes.