I am no expert in electrics but 50 watts is very low and no matter what you do you will not get any more heat from it.
With 2" of cover this seems rather deep my cable is barely 1" deep.
As I see it you have a lot of sand to heat first before any heat gets to where you want it.
I would suggest you raise the cable to something like 0.5"to 1" deep and see what result you get.
I would try dry sand first and then check the temperature, once you have done that, then wet the sand and check it again.
I have had people say that it does not make any difference whether the sand is dry or wet as the heat source will still produce the same heat choose which way you use it.
My own preference is wet, as I get the impression the moisture (water) warms up and radiates the heat better plus it creates a nice humidity so my seedlings are kept just nicely moist, meaning I do not have to water them much thus reducing the risk of damping off.
I have no scientific proof of what is happening but it seems to work for me!
All I can suggest is you try sinking it to various depths.
You mentioned the heat in your propagator being hotter,what you must remember is that there is probably less than 1 mm thick of plastic covering the heating element.
So with this in mind shallower cover can only improve you situation.
Sorry I can't suggest anything better !