2018 Crosstrek Premium, Sunshine Orange
If you feel a need to secure your platform you could run a nylon strap, rope, bungee cord, or similar under the spare tire and attach that to pad eyes mounted to your wood framework. If you wanted something really secure, you could use ratchets from tie down straps on both ends of the strap. As long as the spare tire is secure, it should all be secure and still be easily removable. If the issue is simply that you do not like the "cathank" sound, consider just using small pieces of carpet scraps to cushion where the wood frame is in contact with metal.