If you don't have a sump, then you can't get one that sits underneath the tank. Octopus has recirculating skimmers, which uses a pump to remove the water from the tank and then put it back, but the skimmer has to be around the same level of the tank, and even their smallest recirculating skimmer model would be too large for your tank (the pump itself would take up a huge amount of space in your tank, and is so strong it'd probably suck up every fish in your tank -- I know, I have an Octopus recirc skimmer).
Unless you build a sump, your only choice is to buy one that hangs on the back.