The manufacturer probably says nothing is needed, but you've seen the results! Often, they say setting it in a bed of cement/mortar/plaster of paris is optional, but it really isn't if you want a quality install. You might be able to spray in some low expansion/high density foam with a long hose extension to get it where it needs to be. You'd probably have trouble trying to get an even spread of cement pushed in from the end. If you had better access, you might be able to make a dam and pour in some plaster of paris. I'm not a pro...maybe one of them will have some ideas. It should have been done when it was installed, then it's fairly easy and quick.