I don't think any lightning arrestor will protect your tuner from a direct, or even extremely close lightning strike. It might protect from one some indeterminate distance away. Keeping that in mind, you can use an arrestor intended for coaxial cable. Just ground the body to your through-hull, and connect the antenna leads to the center conductors of the lightning arrestor. A banana plug
on the antenna wire will fit into the center of a coax socket. Downside is there is no positive locking to keep it in place, but you could secure it with tape. Upside is it would be easy to disconnect the backstay antenna from the arrestor and tuner when away from the boat.