Propagation Bench Poser!

I am in the process of constructing a propagating bench (dimensions (in inches) 70 x 21 x 5. Base is 3/4 inch conti board.  (This is a Tony Buckland - Gardeners World design of some two years ago)  Sadly lost my hastily scribbled instructions.
 
I intend to fit the bench with one of your thermostats plus 7.5 metres of your soil heating cable (is this sufficient for the above dimensions ? ).
 
I intended lining the bench with black polythene - what fill thereafter - no doubt sand - builders or otherwise?
 
Your assistance and advice will be much appreciated.
 
John

Many thanks for your enquiry concerning the propagation bench you are in the process of constructing.

By my calculations the 7.5m soil warming cable should comfortably fit the size of bench you are building as converting your dimensions into metric reveals the area to be just shy of 1m² (0.95m²  to be exact) and the length of cable mentioned above will cover 1.2m².     
Opinion is divided as to which type of sand is best to fill your bed with; some say that sharp sand is better as the more angular granules allow for better drainage and there’s also a train of thought that soft sand is the one to go for as the smoother grains sit together, providing better insulation and distributing the heat more effectively. I’ll let you decide but either way, your set-up will be efficient!

I’d also suggest introducing polystyrene to the base of the bench for additional insulation and to get the most from your heating cables, this layer should be sandwiched between the base (black polythene) and the sand. It’s also worth placing some polystyrene up the sides of the bench as well to plug another potential ‘heat leak’.   
Finally, the thermostat! You probably already know this, but when setting up the cables it's best to position the thermostat sensor as near to the area the heat is required to achieve an accurate level of control. You're interested in the temperature of the soil in the seed trays around the root zone of seedlings, so pushing the sensor into the compost should give the best results.
   
I do hope the above is of help but if you have any further questions or queries, please do not hesitate to contact me and a quick visit to the soil heating cable product page will provide you with even more information.
Many thanks once again for your enquiry and best of luck with your propagating activities this year!

Martin