Is building your own inefficient but secure cipher hard? Everyone says you should never build your own cryptosystem. That doing so is hard and the only way you know it's secure is if it stands up to serious scrutiny.
And yet, if you are willing to go way overboard on key size, rounds cpu time, memory etc. Would it not be fairly easy for even a novice cryptographer to build something secure. Especially if using a well understood basic construction(e.g feistal network).
If you are building something based on existing research, then yes, the construction of such a system would not be challenging. As a very basic example, it wouldn't be extremely challenging to implement a system based on the One-Time Pad given that you have a good programming background and assuming you follow the algorithm's stipulations correctly (e.g. avoiding repeated keys for new messages, good RNG, etc.), your system will be provably secure.

However, if you seek to design and develop new encryption schemes, you might have a harder time. You will likely encounter issues in multiple layers starting from the theoretical development and proof of security and all the way to the practical implementation of the scheme. That is, you will have to, firstly, design an encryption algorithm that can be proved to be secure in a mathematical way. Secondly, depending on the complexity of the algorithm that you designed, you will have to be extra careful in avoiding bugs in your code that could allow a motivated attacker to break your crypto system.

Because of these reasons, it is highly encouraged that such algorithms are open: it allows for more eyes to look for both flaws in the theory as well as in practice.

Some resources to expand your research:


https://crypto.stanford.edu/~ananthr/docs/crypto-proofs.pdf
http://eprint.iacr.org/2000/067
(taken from: Proofs of security methodologies)