You just do what everybody else does. Buy the ensurance at the borders or nearest town, when reaching a country you do not have insurance for.
I am in Dakar now by motobike. I bought insurance on the way down for Mauritania and the other countries stated below.
12 euros for Mauritania. Bought in Noudhibou. Valid for 1 month.
44 euros for Benin, Burkina, Cameroun, Centrafrique, Congo, Côte.d'Ivoire, Gabon, Guinée, Equatoriale, Mali, Senegal, Niger, Tchad and Togo. Bought at Rosso border. Valid for one month or 3 month for more euros.
Morocco and West Sahara is covered by your insurance back home. It was in my case.
IMPORTANT: They do not use the Carnet de Passage in Senegal anymore. Stopped a year ago or so. But the automobile organisation back home didn't know that in my case!
Hope this can help.
Around the World Journey | mhoey