Thick resin encapsulization in untraditional mold

I am making an art piece that uses a plastic hollow mold that will be about 4 to 5 inches thick at the widest. I am using a plastic mold of a woman’s pelvic region that is made to display women’s underwear. (https://www.allendisplay.com/Female-Unbreakables-Hip-Form?gclid=Cj0KCQjwibDXBRCyARIsAFHp4frAGuZ_Yt_BCmu2CKqXyXC6BlohPKdn1_edljPuPmZehnvTmsHqaQoaAiEFEALw_wcB)
I used polyester resin (Casting craft) and the measurements of the casting resin to hardener was as close as I could make it (maybe a drop or 3 too many?).
I sanded the inside with fine grain sandpaper (1000 grain) and used mold release. Poured the first layer (about 3/4 of an inch), waited until the resin was gelling, added my small plastic objects, and then did a second pour of about an inch in thickness. The next morning, there was a giant crack, and I was unable to release the resin without further breakage. What could I be doing wrong, or am I using the wrong materials? Should I not be doing this in repeated thin layers? As I mention before, the final art piece will be about 4 (maybe 5) inches thick.