I found this super cool tutorial from soaping101 and I gave it a try.
I did things a little bit different then what I saw in the tutorial... I used a wood box and I filled the bottom with play doh and I filled also all the shells with play doh. I took a picture of the box with the play doh and the shells in the process of placing them, but in the end I modified them some more... but I will post it just so you get the idea... using play doh diminishes amount of casting silicone needed, and I had only 1 kg available.
One thing one should make sure is that one has enough with silicone to cover all the shells.