I believe these are known as side-boards. I'll be placing a 170L (45g) tank on top, so the side-board will need to hold 170kg (374lb) at least. I'll use a thin piece of polystyrene underneath, and make holes for hoses/CO2 etc. in the back section of the side-board. Naturally the unit will need to be perfectly level.
Does anybody else have their tank in a similar set-up? Is this going to present me with any issues?
Many thanks Keith, that's all I needed to know! I'm going slowly with the next project so I may have time to pull my finger out and rescue a nice old piece of furniture..