Heating cable

I have to put a 1 inch layer of damp sand in the bottom of my electric propagator as I used it without once and the heat warped the plastic base.
I will have to venture into the attic to find propagator as I want to sow some chillies and Cape Gooseberries. I haven't used it for a couple of years so I hope it still works. 

Vinny I have had a heating cable ignite peat, the propagator contained peat which was the advised medium to use with a heating cable then, some of the peat dried out and ignited, that's why I have said keeping it moist is very important there is no way I want to risk that again, so I will keep using a 2 inch layer of sand on the base

I have to put a 1 inch layer of damp sand in the bottom of my electric propagator as I used it without once and the heat warped the plastic base.
I will have to venture into the attic to find propagator as I want to sow some chillies and Cape Gooseberries. I haven't used it for a couple of years so I hope it still works. 

Mines been going for 22 years  quite chuffed with that investment.

I want to make one of these - I would like to get my dahlias going so that I can take early cuttings.

I'm not very good at DIY, I'm fine at putting stuff together - it's getting the things I need as I switch off a bit on lists of things to buy.

Can you point me in the right direction?
I've enough space in the Green house, I have a small section partitioned off....I just need to know what to get to make it.
All in plain English.

Scarlet first you need a electric supply in the greenhouse, I made a wooden box 4.6ft. X 2ft. X 9inches high, the base of mine is made with 9mm. plywood so I added a sheet of polystyrene below that as insulation, line the box with polythene this is to keep the infill damp so you want it watertight, if you puncture it, seal it with waterproof tape, I use concrete sand as the infill medium and add a 2inch layer over the base then run a soil heating cable mine is 6mts. long (follow the instalation instructions on the box) then cover this with another 2 inch layer of sand, the temperature should be controlled by a thermostat and nowadays you can buy them with a socket into which you plug in the heating cable and it has a temperature probe fitted as well and you can push this into the sand or place in a pot of compost sitting in the propagator, once ready pourwatering cans of hot water over the bed, doing this gets the temperature up quicker, I strongly recommend sand as other flammable material can dry out and ignite, and always keep the sand damp which helps the heat distribution you should be able to buy the cable and thermostat in any GC or you can buy them on the Internet I have my propagator covered with a poly tunnel but will be removing that as it causes too much moisture in the atmosphere of the box and will replace with fleece
List of materials
Wood or plywood to make the box
Sheet of polythene
Washed concrete sand 4bags
Heating cable
Thermostatic controller
And most important, electricity

Thank you - I'm going off to do the buying/cost research