Right, so now you have to take your plywood and line it up perfectly with the body. If you left 'safety' room on the plywood, then try to get the 'body' in the middle of the plywood. You want a strong wood glue, but if you are doing the extra secure method below, then it doesn't really matter (but you will need a strong glue later in the project).
Hammer nails in through the plywood and into the 'body' at regular intervals. Use however many you think you will need to make sure it never comes apart.