Well Soeren, I can see 2 types of lava, the first would be the agglomerated rocks when it('s dried up. i think you could easily create this with thinned down plaster and rough cat litter -by binding together the big grains with that watery plaster.
If what you want to do is VERY smooth lava, just like when it flows, you must absolutely not touch the plaster with any instrument.
Look at JBA's Spaceman 3 build. at one point he pours some thinned down plaster through a hole in the boat using some paper sheet, you must d o the same and let the diluted plaster spread where it wants to go. And once the first coat is okay, applying an even more diluted coat over the first one to do some other run off etc etc..
the trick is :
-use diluted plaster
-use some device to pour it on your terrain
-DON'T touch it
You are pretty right when it comes to the amount of scratch builders here.
Speaking of which.. it's time to build that round portion of hull.. quite round indeed.
Here is the plastic shape once built. Thanks God for Tenax

Let's straighten the angles with some magic Sculp. The pmroblem is that the magic Sculp doesn't glue so much to plastic, yet I sanded it roughly so that it would attach better but well no. I had later to use some CA glue to straighten the joints

Well it took me more than 1 hour to trim this thing!
in the end very few of the original shape stays visible.
