If this tank is only going to be a temporary home, for less than 2 years, your standard Snowflake or Zebra ell will work for a while. Not forever, our snowflake outgrew our 60gal, years and years ago (early 90's)....
The obvious recommendation which you'll probably get a couple times is the Golden Dwarf Moray, Gymnothorax melatremus. Reaching a maximum of 15", would seem right at home in a 45gal, which he would be. Except their most for the typical aquarist. They're an extremely shy species. Most owners report not seeing theirs much at all. If you want other fish as well, this will increase their inherit shyness. They are always typically about $150-$300 depending on vendor.
Just asked for the budget, because on a different WYSIWYG site, the Skeletor Eel, is starting to regularly appear. Max length of 24" or so, the would fit great too, unfortunately they command a $500+ pricetag.