def mount on boards - if you see these installed in outdoor toilet blocks (a favourite location) you'll see the scorch marks on the wall where they have radiated heat over time .... replace the board easy enough ....
Since its an engine compartment that you are interested in heating I would go for a 60 Watt tubular. My mate has a Hunter Horizon with one in the cabin and its always warm and dry for minium cost.
bought 3 60w tube heaters from TLC just before Christmas. http://www.tlc-direct.co.uk/Information/default.htm
£42 including delivery and arrived before I could tell my wife that I'd ordered them.
I made ply bases to mount them on and leave them on 24/7
at 60w each they don't throw out much heat so ignore the rubbish about timers and thermostats.
I recant. I am wrong. I deserve to be banished. Even at the 180W my heater draws, and even at the price the yard sells electricity, a time switch or thermostat is not cost effective!