Making technical reports available
If you are affiliated with an academic institution, many have some sort of mechanism for publishing tech reports, e.g. via DSpace.  This is the ideal solution, since it is designed for exactly the sort of publication that you are looking for.

Failing that, I would suggest looking a little bit closer into arXiv.  Many of the topics are quite vague, and depending on the topic, your work might well fit into something like cs.IT (Information Theory) or cs.SY (Systems and Control), or a number of other areas.

Finally, a possible fallback is to simply publish via a fairly stable repository like GitHub, which means it is at least not dependent on your own server, but an institution that is intended to be maintained over many years.