I think I found a much easier work around. Hopefully this helps anyone else who stumbles across this question:
\begin{align}&E^2 + \frac{1}{4}(-2E^2 +2B^2)\\&= 1/2 (E^2 +B^2)\\&= \mathlarger{\boldsymbol{\varepsilon}}\end{align}\begin{flushright}\qedsymbol\end{flushright}