For the easiest, you can just apply multiple encryption algorithms in succession (of course with different keys). Although the algorithm of AES is considered safe, it can be broken through a side-channel such as a backdoor, which secretly stores keys used somewhere. But if you apply another algorithm after AES, be it ChaCha20 or Blowfish, it can only gets reinforced.

Another trivial way to safely roll out your own encryption is to increase the number of rounds in ciphers that are considered safe. The increased number of rounds only strengthen the algorithm. And it's just changing a few magic numbers in the source code - you can get extra security for little expense of time.

Both methods provide esay-to-implement ways to safely 'invent' a new encryption algorithm without a proper knowledge of cryptography. If people start doing any of the above regularly, it would be a headache for those enjoying to exploit vulnerabilities in common crypto implementations.