This is one of the nastiest problems in applied electronics to solve, and many people have made a career out of solving it.
Don't waste your efforts on fuses. By the time they get around to doing anything, the circuit is already fried.

The problem breaks down into several pieces - limit the voltage excursion as quickly as possible, limit the current flow, and direct it somewhere safe. You will be challenged to do this effectively with a MAX232 because it has a built in switching power supply to enable driving the outputs to double the supply voltage, so there is no solid power rail to clamp to.
Board Layout will be absolutely critical too. Honestly, this won't be solved by a couple clues on a user forum. If you want to avoid the cost of external protectors, you need someone to go over the complete design from this perspective, including silicon technology choices, reference points, trace widths and spacing, application specs (cable lengths and types) and so on. You are as likely to make it worse as better without a holistic approach.