OK here's a bit of info on converters.
For years, linear voltage converters were used to convert 24V D.C. to 12V D.C. But linear converters are technically quite primitive,being in effect, little more than a resistor. This method of reducing voltage has the disadvantage of being very inefficient and as a result at least 50% of the input power is wasted and given off in the form of heat. To deal with the heat a heat-sink is required, increasing the overall size of the converter and restricting its positioning.
Switched-mode converters however, work in a much more sophisticated and efficient way. At the input is a field effect transistor(a solid state switch in basic terms) which converts the input voltage into a square wave. The square wave has a 50% duty cycle (50% on, 50% off) and, when this signal is put through a rectifier with a capacitor, the output voltage is 50% of the input voltage. Because the resistance of the switch is low, power loss is reduced and efficiency is high. Maybe as much as 95% efficient at load.
Other benefits of switched-mode converters are, smaller size due to less need for heat dissipation,greater flexibility in choice of installation location,increased battery life,longer life of the converter itself,low standby current drain.
A common problem with switched-mode converters in the past has been that they create significant amounts of radio interference so you need a good quality converter that will be heavily filtered and not produce interference.
It's important to have a converter which is suitable for the use to which it is to be put. Trying to draw more power from a converter than it
is designed for will result in the failure of the converter and possible damage to the equipment being powered.
Most converters use two figures to describe their performance, 8/12amps for example. The first figure is the power which the
converter can supply continuously. The second is the power which can be supplied for a short period of time, usually a few minutes
and is called the peak power rating.
Make sure that the total power consumption of all appliances which will be used at once is within the continuous power output
capability of the converter. For example, to run a 12-volt TV which uses 48 watts of power will require a converter with a minimum
rating of 4 amps. 48 watts / 12 volts = 4 amps.
Bear in mind that some appliances - TVs and compressor fridges are good examples, require more power when they start up than they do
when they're actually running. A TV which consumes 48 watts in operation may take 100 watts or more for just a fraction of a second when it's switched on. The same applies with a compressor fridge. So when choosing a voltage converter, make sure that it has a peak power rating high enough to cope with start-up current requirements.
So find yourself a decent switched mode converter, probably around 20amp rating and a small aftermarket automotive blade type fusebox with a cover.
You should shop around for a decent price but here's a couple of examples.
http://www.thetoolboxshop.com/0-578-18-24v-to-12v-durite-voltage-converter-18a-3518.html
http://www.thetoolboxshop.com/0-234...ard-blade-fuse-box-with-clear-cover-1484.html
lets see how they are when they get here 
