Trying to figure out how to join without a seam line?

Here is how I would go about it. Run your glue down the seam as normal (Tenex, Tamiya extra thin) let it set a few seconds then squeeze them together letting the softened plastic ooze up. Wait for it to completely dry then take a brand new #11 blade and carefully scrape the ridge of plastic off. Go back in with a needle file to fine tune any remaining spots.

What Rambo said.  I use weldon #4 for stuff like this.  Hotter than tamiya.

   I agree, no "easy" way out of this one. The extra thin cement and squeeze method is probably the easiest, sometimes I just keep brushing glue into the seam after its together however I also usually still end up with some clean up.

    Good luck and happy modeling.