My TR3A is an early one - TS 27489 LO. I still have the bits and pieces of my original trunk (boot) lid seal in my garage. The seal was originally sealed into the channel with a rubber cement. There were two joints, in the upper right and left corners, just to the right and left of the hinges. When I put a new one in in 1990 as I was finishing my restoration, I did exactly as Harry describes with a stepped overlap joint. By matching the steps, the top of the seal stayed flat and flush.
I put it in and thought that someday, I'd get around to putting in the rubber cement, But I never did. And I found out that it can't go anywhere with the lid closed. And when I get to a concours show, I can whip it out, chean it like new, wipe out the channels and then put it back - all in a jiffy.
However about two years ago after driving in a rain storm, I found that I was getting water in the boot. Because the rubber seal had never been glued into the channel. it was receding or shrinking in length and now it was getting shorter at both ends of both pieces, so the stepped joints were leaving gaps which allow the water to get in. I plan to buy a new length of seal.