I need to build a pillar to house an ESB meter box. My design has been approved by ESB rep and electrician and I am about to buy the material but as this is my first time doing any kind of masonry work I wanted to get opinions from more experienced DIYers.

I designed the pillar to be 670x670mm & 1950 mm tall with a 600mm deep footing block which extends 150mm from each sides. The pillar is made of cavity blocks (grey) up to the opening of the ESB box (1100mm high) and then made of 4 inches concrete blocks (yellow).

I have never used cavity block and I am wondering if it's ok to leave them empty or do they need to be filled with sand/gravel or concrete.

Let me know what you think please :)