i use the little cable clips you can buy for securing telephone cable in position
they come with a little panel pin nail,i just replace the nail,with a small self tapping screw
put the probe cable into the clip,then screw the clip into the position you require it to be in
i use 1 cermaic bulb,bulb guard and thermostat in each tank
imo theres no need for multipule heat sources mate
you can see how i position the heat source in this picture,i place the heat probes,4 to 6 inch back towards the cool side,6 to 8 inches below the bottom of where the heat guard stops
i've moved all my probes since that picture was taken.i will try and get some pic's of there new positions for you
they are attatched to the back wall,5 inch towards the cool side,from the heat guard,6 inches down,from the level of the bottom of the heat guard
cheers shaun