I don't like inverters for the efficiency reasons. Although they get warm enough to be effective dew removers from eyepieces. But then the laptop fan is usually good enough for that I suppose
Anyway...
I use a 12v to 15-24v type device, not from DSE but from
http://www.12voltshop.com.au
I'm not sure if there's a difference in quality. I bought mine about 3 or 4 years ago, still going strong. I think it was about $120 but can't remember clearly.
Perhaps you have some other problem with the power setup of your equipment which is frying the converters? Might be worth investigating before you blow another, or an inverter.
The only alternative to the above which I can think of is to have multiple laptop batteries. More costly and less adaptable.
Roger.