Generating an organic boulder geometry

Hello, 

I've been a little dormant with my grasshopper skills for about a year so I'm having a bit of trouble figuring out how to approach this little problem. 

I'm trying to create an organic boulder like geometry. I'd like to be able to generate various base shapes (random or otherwise) and generate various bulges and distortions on its surface. 

I guess a great precedent for what I'm trying to do is the Ordos Museum
by MAD (see below). 

In the 'boulder.gh' definition I've taken my best shot at creating a base blob for the boulder by distorting points of a box but this is far from ideal and doesn't really produce the results I'm after.

Any tips/hints/suggestions would be much appreciated. Even just advice on how I should I maybe approach this would be useful!

Thanks for your help!