Answer:
Either make it regular by dividing it into smaller objects and doing maths
or better still and more fun....
Fill a bucket with water right to the top carefully measuring how much water it takes
then dump the object in and watch it force the water out.
then re measure the volume of water left in the bucket and do some subtraction
presto a volume for whatever you dumped into the bucket.
This only works if your object doesn't mind getting wet and doesn't soak up water and doesn't float on water....