Plastic filaments seem to work well, particularly when combined with chemical cues.

Doroudi, M. S. and P. C. Southgate (2002). "The effect of chemical cues on settlement behaviour of blacklip pearl oyster (Pinctada margaritifera) larvae." Aquaculture 209(1–4): 117-124.

P.C. Southgate, A.C. Beer (1997) Hatchery and early nursery culture of the black-lip pearl oyster (Pinctada margaritifera, L.) J. Shellfish Res., 16, pp. 561–568

it seem many marine bivalve species looking for  filaments materials when settling, each species might have specific one.  

Plastic material work well, preferebly with a biofilm which grows after having submerged at least for a week. Take into account spawning season.

Monteforte, M., Kappelman-Piña, López-Espinoza, B. 1995. Spatfall of pearl oyster Pteria sterna on experimental collectors at Bahía de La Paz, South Baja California, Mexico. Aquaculture Research, 26: 497-511

Monteforte, M. 2005. Ecología, biología y cultivo extensivo de la Madreperla de Calafia, Pinctada mazatlanica (Hanley 1856), y la Concha Nácar Arcoiris, Pteria sterna (Gould 1852) en Bahía de La Paz, Baja California Sur, México. Post-Doctorate Thesis. Facultad de Ciencias Biológicas, Universidad de La Habana. Cuba. (Google it, free for download but without pictures and tables. I will upload the complete PDF in Research Gate)

P.D. Google for Pinctada imbricata Pteria colymbus Lodeiros Venezuela. He has several papers on spat collection and extensive culture with these species

we are producing pearl oyster larvae since 2009 and for the settling of spat we used plastic pipes pieces semi round /u shape or plastic sheets ,