Some smaller species of monitor? Tree or Timor perhaps?
All good suggestions, and yeah I was thinking it would be on the small side for them.I'd go with a medium-sized arboreal lizard such as a green basilisk (Basiliscus plumifrons), brown basilisk (Basiliscus vittatus), or frilled dragon (Chlamydosaurus kingii), or a medium to large arboreal snake such as a carpet python (Morelia spilota).
It's too short for either of those monitors; they prefer taller enclosures and feel safer up high.
A dwarf monitor pair...temperary thou.What do you want to put in there?