kasko

No suffix here. Kask- is a single root. There is no -ask- in Esperanto, and kas- is not a morpheme either, so the word cannot be split and has to be learnt whole. It is worth saying because the language trains you to look for seams and the looking has to be switchable: the majority of Esperanto roots are indivisible international borrowings, and only a minority are assemblies.

Ancient and modern, one word. Esperanto uses kasko for the Roman legionary's helmet, the medieval knight's, the soldato's steel helmet, the cyclist's foam one and the builder's hard hat. If you need to be specific, compound: batalkasko, motorciklkasko, laborkasko, sunkasko for a pith helmet. Kaska is the adjective, and kaskita describes somebody wearing one.

Nearby words that are not it. Kaso is a till or a cash desk, kesto a box or a chest, kastoro a beaver. None of them shares a root with kasko, and the resemblance is pure accident. The English pair casque and cask, on the other hand, really are both casco: a cask is a helmet-shaped vessel, which is a nicer piece of etymology than it has any right to be.

Wearing one. The verb is porti, the same one you use for carrying and for clothes: porti kaskon. Surmeti la kaskon is to put it on and demeti to take it off. Modern usage has fully absorbed the safety sense, so kaskodevo (a helmet requirement) and kasko deviga turn up in road-safety writing without anybody flinching.