mo (lspace=1em rspace=2em): $\text{\_}+\text{\_}$

embellished mrow (lspace=1em rspace=2em): $\text{\_}+\text{\_}$

embellished munderover (lspace=1em rspace=2em): $\text{\_}\underset{\text{|}}{\overset{\text{|}}{+}}\text{\_}$