Adding small holes at each end to keep from spreading is a good idea. Eternabond or Flex Tape over the crack will keep dirt and water from getting into the crack. Dirt in the crack when it flexes will slowly make the crack larger. Even if not all the way thru, water can migrate thru the crack and will enable further deterioration of the fiberglass.
As noted, putting a fiberglass patch over the crack is the best solution - but is more work and cost. I never enjoyed working on fiberglass over my head...
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S.E. Mich. Flagstaff 26FKWS / 2019 F-150 3.5 EcoBoost SCrew Propride