Question is - travel alone or with a passenger?
If alone, nothing simpler than a proper bag mounted on the passenger seat part. It can be expanded left and right adding side bags secured well with a couple stripes.
I traveled Mexico from Cancun to Copan Ruinas in Honduras, Guatemala stopping again at Antiqua for 2 weeks, Belize orange Walk back to Mexico and Chetumal, and from Valladoid to Merida, Campeche, Puebla, back to San Cristobal and Palenque, and lots more, from Vancouver Canada and back, 8 months, with a small backpack and all 3.5 k dollars. All you need is 3 pair of socks, 3 underwear, 2 shirts, fleece, windbreaker, and that's all.