I have used GE silicone 1. (Without any mold inhibitor). It takes a while to cure but it is generally considered safe. (Wait a few weeks. It takes a while for the smell to dissipate).Remember to carve the PUR foam or else it won’t adhere. I have never used flex seal so I cannot comment on whether it is safe or not. The build is taking shape nicely. I look forward to seeing it planted.