Find a CFG for the set of prefixes of a CFL
If you know how to do Suffix, then you also know how to do Prefix. Explicitly, show how transform a context-free grammar for a language $L$ to a context-free grammar of its reverse $L^R = \{x^R : x \in L \}$, where $x^R$ is the word $x$ in reverse (for example, $(for)^R = rof$). Then use the fact that
$$ \mathit{Prefix}(L) = \mathit{Suffix}(L^R)^R. $$
If you unroll this definition, you will actually get a direct construction for Prefix.