Also, shells that size are heavy enough that they would drag down even firm breasts if just glued to them.
In the movie : http://images2.wikia.nocookie.net/__...aidFriends.jpg
There's a strap that goes behind the shells. So you could use the front of a strapless bra, attach your own strap, and do those clear straps over the shoulder, or nude colour ones - you can even use makeup on fabric ones to get them as close as possible to your skin color. Normally you want the band doing the supporting - so this is only a good solution if your breasts themselves don't need "support".
If you need more support, you could use a strapless bra with its original back and have the bra be either purple, partially painted purple, or nude with a different strap over it. With or without straps like above.
With some wig hair over the shoulders the straps would be hidden - and mostly you don't even see the band on a bra when people do it that way because the shells are so large.
If instead of real shells you cast the shells and made them out of latex they would be a lot lighter, not fragile, and be able to be glued on more securely with a decorative back strap. I would still wear those petal pasties under it - to protect the more sensitive areas from the spirit gum or other adhesive and just in case.