I would think plain old Super Glue would work fine. Another good crafting glue is Mod Podge, it dries clear. Sign up for Joann's Craft Store coupons both in the mail (get the application at the store checkout counter) and online. They always have the 40% off coupons and then some.

Sal

Thanks Craftylady,
I will check out Mod Podge. I have Aleene's Tacky Glue, but maybe that's more for paper crafts.
And yes, I do get the JoAnn mailer; love those 40% off coupons!

You also may want to consider adding E6000 to your adhesive collection. Makes a very strong bond and good for heavier objects.

There are lots of great glues out there that I'm sure will hold your shells in place very well.
Question: How are you going to clean them when they get covered in dust, especially if they are glued in place?
If you put a piece of plexiglass over the front of the printer's tray you would have a much cleaner display over time, you wouldn't necessarily need to glue the shells in either so you could rearrange it if the mood struck...
just my 4 cents
Suzan J