Stronger encryption is the only answer, but I'm afraid merely XORing each byte with a "key" isn't going to keep any determined cracker out for very long. That amounts to a key length of just 8 bits, or 256 values to test in a brute force attack (assuming you were planning to XOR each byte with the same value).
However, if your key is randomly generated, and the same length as the ROM data...
However, if your key is randomly generated, and the same length as the ROM data...