So I want to build freestanding wall with high thermal mass at the end of my driveway (North facing with good sun exposure)

Purpose is to grow passionfruit vine.  My neighbours have one doing very well, on a North facing wall of their (brick) house.  My assumption is that the thermal mass of the brick wall and the shelter of the eaves stops their passionfruit succumbing to frosts.

So I want to replicate this.  I can use some corrugated clear plastic to provide eaves.  The tricky bit is to build a high thermal mass wall 1.8 high cheaply and securely.

My only idea so far is to build a concrete block wall and fill it with sand.