I made got the last part of the device. It gave mean the going together message, but the big beholder did not show up. Any thoughts of how to force it?

Did you look around the area a bit, in case he spawned out of your sight?

Beyond that, I'm sure you can use console commands to force a spawn; not sure what the exact command would be, though.

He spawns by the entrance to the chamber where you find the second half of the rod... so you'll only see him when you head towards the exit.

However, if you save your game after you assemble the rod but before he arrives, BEWARE! If you reload that save, the spawn trigger gets broken and he'll never appear, forcing you to reload an earlier save. You should always save before opening the container and not overwrite that save until after he's dead.

If you think that's what happened to you, then your best bet is to reload an earlier save.

Yep. I am pretty sure that is what happened to me AND I think i ended up overwriting my earlier save. I tried to use the old debug codes to see if I could force a respawn, but they do not seem to be working.

Try using Shadowkeeper to remove the completed rod from your inventory and add the 2 parts back in (though I recall the rod is auto-assembled when you pick up the 2nd part, it might work though)
Also check the global variables to see if there's one about the assembled rod and if so, remove it.
(obviously do this on a separate save name to test, which shadowkeeper should do automatically).

Other than that the only thing to do would be to use Near-Infinity to edit the area file of the saved game to replace the 2nd rod half and reset the spawn (but I've not got it installed at the moment so I can't check which bits to edit)