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If you're not looking for strength, and just basically trying to fill a space, there are two-part rubber compounds (similar to "Spartite") - Devcon is one maker of such material. You mix two parts into a syrupy rubber soup that pours into place and self levels and then sets..
Make sure the area to be filled is as clean as possible for good adhesion.
Getting it out later if necessary would be a chore, so be sure this is something you want to be "permanent".