It really depends. If the tolerance isn't too tight, simply placing it on the shelf is what most people do. Without a Router, it is difficult to make recesses for the Flex. The simple act of sliding the tubs over the flex without a lot of downward pressure isn't too big a deal.

Use Aluminum tape to hold it down. About $6 for a large roll at Home Depot or Lowes. It conducts the heat well and keeps it tackiness as well.

Don't forget to make sure you have somewhere for the thermostat probe when all the tubs are in place. And have fun with the build!