I once used a Big Stick with the whip cut down for VHF Lo Band and it worked excellently. Use the formula 234/f (in MHz) and take a fine hacksaw to it, then you'll have to put a somewhat larger boot on the end to prevent moisture ingress and fraying of the fiberglass. Alternately you can cut down a 102" steel CB mobile whip; but in any case use a stainless steel split ring washer, the whip has a nasty habit of loosening without it.
While you're at it you really should coat it with a good quality marine fiberglass paint, they weather badly and get all fuzzy as the glass fibers are exposed. Another thing, since FM is nearly impulse noise immune it's about all that POS antenna is good for. I'll never again use a fiberglass CB antenna, being non conductive they are very prone to precipitation static due to charge build up on the fiberglass like rubbing it with a cat. That's why I replaced mine with a Ringo, being aluminium and "DC ground" the static hash went bye bye.