Small hard 3-SAT instances
If the number of clauses is very small, then there's no clause/variable ratio that makes 3-SAT hard.  3-SAT with &lt; 20 clauses is easy.  It's asymptotically easy (technically, the running time is $O(1)$) and easy in practice (using off-the-shelf SAT solvers).

One way to derive hard instances is based upon cryptography, where you convert the problem "find a value $x$ whose SHA256 hash starts with 128 zeros" to SAT, or "find a 128-bit AES key $k$ that encrypts a fixed 128-bit string to another fixed 128-bit string".  However, these are still going to have thousands of variables and tens of thousands of clauses, at minimum.