What are you using at the moment? Whichever manufacturer you are currently using for your container wax, most likely makes a votive/pillar blend as well. You can usually add more FO with a pillar wax than you can with a container wax. You will just have to experiment the FO load. Try 2oz per lb of wax when making tarts, and see if you get seepage. I wouldn’t recommend using a container wax with the clamshells, its can be hard to remove them since container wax does not pull away from the sides.