Sometimes, if the booster is right next to another seat (as in a tight 3-across situation), the child really can't buckle the seatbelt him/herself, no matter what. An adult may have to tilt or slide the booster out a bit, temporarily, to get their hands in enough to fasten the seatbelt, and then shift the booster back into position. My 6 1/2 y.o. can snake his hand down to release his seatbelt, but there is no way he can fasten it himself. For reference, he's in a Parkway booster (which has no armrests, and is one of the easier ones to buckle), right next to his little sister's RF Radian. I have to get both my hands down between the seats in order to buckle; it was difficult at first, but I've gotten pretty quick at it with practice.
Just saying, there may be no solution that allows the child to do their own buckling... but you get used to that pretty quickly, and it's not so terrible.
Katrin