Mixing nightstands can be tricky because you almost want them to be completely different, anything close actually looks a bit worse. I would look at side tables for a real eclectic look. Unfortunately side tables rarely come up to 27". This first one does:

Since the Portman nightstand is 36" wide and 26" tall. I think you will need a nightstand of similar dimensions and in a similar style for it to look okay. I would suggest a single drawer nightstand with legs. Wood with medium and dark variation would look nice.