Set the lid up so the bottom two screw holes on the hinges screw into the back of the original freezer's top two screw holes, then use the top screw holes in the hinges and attach them to the collar. After that, a bead of caulk should hold it down tight.
Please disregard my repeated use of "screw holes"
edit: just saw you're using 2X6. I used 2X4 and this was no problem. Not sure if extra height might affect this.
Last edit, I swear: