I know this sounds odd, but you could try filling some ballons with water (only fill them a quarter full so there is no danger of them bursting) and putting them in the bag.
Water is less dense than sand so is slightly lighter - plus when you hit it, it does ‘give’ a little (like punching a person)
MY FATHER SOVIET MINER WITH ORDER OF LENIN
HE TAKE GERMAN WIFE AFTER WAR
CARRY HER FROM BERLIN TO ODESSA
MAY FATHER STRONG MAN, CAN CARRY STRUGGLING WOMAN 2 MILES
HE BUY HAT WITH FRUIT FOR WEDDING
WE HAVE MANY GOATS AND MILK WOMAN FOR FOOD
HE BORED WITH HER 1982 AND HIT HER ON HEAD
HIT HER SO HARD SHE FORGET ALL ABOUT LIFE
NOW SHE DEAD, WE SKIN AND WAX BODY
LITTLE BROTHER IS ARMENIAN WRESTLER AND NEED PRACTICE DOLL
USES MOTHER
HE LEND MOTHER TO YOU FOR TWO GYPSY CHILDREN
YOU CAN POST TO ME, ILL MITCH, AND I SEND MOTHERBAG