If you want to securely attach the receiver to the rack itself, the 707 may be compatible with the Onkyo/Integra rack-mount brackets that attach to the screw holes in the side of the chassis, such as the IRK-175 kit.
I don't know if the holes would line up properly, though, or if the 707's chassis is designed for that type of mounting. You'd want to research that, or maybe give Onkyo support a call to ask, if nobody else here can clarify this.
If it will work, don't buy the kit direct from Onkyo. They want like $200 for them. You can usually snag them on eBay or Audiogon for considerably less, as the kits are included free with Onkyo Pro processors and many users don't need them.
If there is no kit compatible with the 707, you can always mount a shelf to sit it on, as mentioned above.