Have you considered a hot bed.
I found that for around the same price as a top of the range propagator I was able to build a hot bed that is abot five or six times bigger than standard propagators,
Here is how I made a thermostatically controlled heated bench for rooting cuttings or germinating seed that require a temperature of between 50°/60°F (10°/15°C)
The 2.1m x 0.75m (7’x 2.6’) hot bed I made is; simply a box formed with treated timber and lined with 1” (25mm) thick polystyrene sheets, and filled with river sand.
Materials
100 x 25 mm treated timber edging= 2@ 2.1m long
100 x 25 mm ditto end boards = 2@.75m
100 x 25 mm ditto base boards with 50mm gap between each board = 12 @ .75m
Note; Obviously if your bed is of a differnt size then you will have to re-calculate the materials25 mm thick polystyrene sheet 2.1m x 0.75m
River sand as required approx 50 kg
40ft soil heating cable
Rodstat
Nails & screws as required.
Method;
Form timber box from treated timber and put in place.
Note the structure including the sand is quite heavy so it needs good bottom support.
My set up is placed on benches at each end.
An alternative could be 100mm or 150 mm concrete blocks the choice is yours.
Line the bottom of the timber box with polystyrene then add approx 25 mm of sand.
Lay out the cable in a spiral fashion on this sand bed.
Drill the edge board at a convenient point and insert the rodstat, ensure that it does not touch the cable.
Top up the box with the remainder of the sand.
Connect the rodstat/cable to the power supply and that is it!
Note; To prevent the wet sand drying out too quickly I cover it with a sheet of polythene.
By doing this I find the condensates that form under it, do not evaporate, this keeps the sand moist at all times,
thus giving me good heat distribution.
There are a few photos of my set up here;
http://www.thegardenersalmanac.co.uk/Data/Greenhouse%20heating/Greenhouse%20Heating.htmI hope this is of some use to you.
If you require more info get in touch...Tg