Any kind of seeds might work too---strawberry seeds, crushed grape seeds, raspberry seeds... they sell a lot of these as well as good ol' pumice at Brambleberry but the seeds you might be able to get for free out of food!
You can also fill up your mold and then weigh the contents to know exactly how many grams/ounces your mold holds to ensure you get enough soap to fill it to the level you want. Calculators like thesage.com will also re-size a batch for you after you input the ingredients.