Fe$4L6$ tetrahedral cage $1$ undergoes post-assembly modification (PAM) via a Diels-Alder cycloaddition of the anthracene panels of the cage with tetracyanoethylene (TCNE). The modified cage $2$ possesses an enclosed cavity suitable for encapsulation of the fullerene C$\mathrm{<mrow\; data-mjx-texclass="ORD">60</mrow>}$, whereas original cage $1$ forms a unique covalent adduct through a Diels-Alder cycloaddition of three of its anthracene ligands with C$\mathrm{<mrow\; data-mjx-texclass="ORD">60</mrow>}$. This adduct undergoes further PAM via reaction of the remaining three ligands with TCNE, enabling the isolation of two distinct products depending on the order of addition of C$\mathrm{<mrow\; data-mjx-texclass="ORD">60</mrow>}$ and TCNE. Modified cage $2$ was also able to bind an anionic guest, [Co(C$2$B$9$H$\mathrm{<mrow\; data-mjx-texclass="ORD">11</mrow>}$)$2$]$\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$, which was not encapsulated by the original cage, demonstrating the potential of PAM for tuning the binding properties of supramolecular hosts.