Obscure disclosures are available to the public, yet nearly impossible to find. There are numerous possibilities in obscure disclosing, of which I will give you some to consider.
The non tech local newspaper
This used to be the trick in obscuring disclosures, however more and more newspapers, even local ones are being scanned and becoming digitally available.
The uncommon foreign language
If the local newspaper is still to open to the public, it might be sensible to translate the invention to a less available language, and then have it printed in the local newspaper. It may be odd, that inside a local newspaper, suddenly a technical document in for instance a native African or native American local language is published.
The obscure library
A book or a magazine, delivered at a department library of an unrelated university sub department may also do fine. If your invention is for instance about making cheese, submitting a paper at a library of aerodynamics will render this publication by nature hard to find.
The third parties observation
The reason to submit the documents hand written, is because more and more the documents filed at the European patent office are scanned and the characters are optically recognized and made available in searchable documents. Hand writings are simply a lot harder to make searchable.
Be however careful, that the obscured document is not so obscure, that in fact no-one can find it, no matter how hard they try. Than the obscured publication, may after all not be a publication at all.
I wish you all happy inventing,