Hello,
As you know when a solid piece of material is damages, the structural integrity of the item is damaged, and it doesn't help that ABS is a very unfogiving material. I see Cartop Carriers all over my city with scuffs on them because the things are simply easy to forget that they are even up there!
In my research on your issue I found a material that should fix your cracked carrier and ensure that you will have many more years of enjoyment with it.
To answer your question, Fiberglass is not the proper material to use on ABS. Fiberglass is essentially a "Band-Aid" on ABS because the bond will only last a couple of years.
For this job you will need to use a compound called "Re-Plast" manufactured by the automotive parts company Wurth.
The benefit of Re-Plast is that it hardens into a plastic that bonds well with ABS, and it bonds FAST! So work quickly.
The Re-Plast will give your carrier a "scar" but should patch up your crack and increase your structural integrity. The good thing about Re=-Plast is that if you want to, you can paint over it to match your carrier's coat, to conceal the "scar" even better!
You can order Re-Plast directly from Wurth on their website link that is found below.
https://shoponline.wurthusa.com/wurthusa/servlet/CyberVendor/category/G4240843/catalog/group.jsp/
If this does not solve your issue, or if you have any more questions, please leave a comment on this thread and I will be more than happy to assist you further!
Happy New Year!
Feedback is appreciated!