You could try & fashion something from spare transparent sprue, You would want to start with something slightly larger in diameter than you require - the final parts of sprue which lead to the canopy or whatever are usually thinner & may be more suitable.
Get a short length of the sprue & clear away any flash or edges on it & then stick it into the chuck of a power drill / craft tool. Rotate the tool at low > medium speed & use some fine paper to create the profile you require. When you get something looking like what you want, switch over to a fine abrasive powder / liquid (even toothpaste) & finish it off to give & smoothness / shininess that suit you. Then cut the tip off (rolling it under a very sharp blade - like stripping the outer sheath of a cable) & voilà.
If you really want to go to town, before you cut the tip off, make the cut to give you a slightly longer piece & use a fine drill to drill a recess into the back of the part (going towards the front), then do you final cut & you will be left with a recess which you can fill with chrome paint / foil. this will give a more realistic look & still leave you with a surface suitable for gluing. whatever you do I would use PVA / wood glue (Elmer's?) to join the part's - it won't leave such visible mess.
A different method would be to try & form the seeker from fine drops of PVA.
Having said all that, the part may be available on the aftermarket, or someone may chime in with a far easier way to do it!