Take off the Y and just snap one of the Loc-Line flexi arms directly to the back wall instead of the Y. Then you can flex it down and use a shorter piece of hose as long as it is close in diameter. If the hose is just a little small you can use a hair dryer on high for a minute or dip the end of the hose in boiling water for a few seconds to soften it and try and slip it over the loc-line. As it cools it will be a tight enough fit that you won't have to worry. On the barb/PVC side I would probably suggest a plastic hose clamp as an extra precaution, but I really can't see a way it would come off on its own if the hose it cut perfectly to length, which with the loc-line arm it doesn't have to be that perfect because you can adjust it a little.
Only issue I can think of is the hose not being thick enough and a rock possible crushing the hose if you rest a rock against it.