Your next challenge:
Do another mould. In it, place rocks so as to form cubes - you are essentially "anti-digging" rooms by constructing rock rather than digging it out. Do this until you've designed all your "rooms". The mould should be held up by supports linked to a single lever. To prevent spillage, dig a trench beforehand, in which you'll place the supports so magma can't seep through. Do the same for your "rooms" by placing supports at the bottom of it. Therefore, your stone structure will be supported by nothing but these removable supports. In the chambers, pour water until full.
Around the mould, leave two or three spaces alone and build an external mould, also on supports but linked to a different lever. In the in-between space, pour water.
Now that you are completely ready, pour your magma, all of it, in a single go.
Here's what you've got: a big block of magma surrounded by an outer later of water, and surrounding a fort-like structure that has been completely filled with water.
Pull the lever!
Your supports will vanish. The inner mould will collapse along with the fortress structure, releasing the water within and without, and creating a perimeter of obsidian - but the inside of the obsidian block should be magma! After all, it hasn't been touched by water.
Pull the second lever to take out the outer mould and let the extra water spill away.
With this done, pour some water on top of your block to cool whatever magma was left over there, and move in.
You should find a lot of rubble inside an obsidian network of rooms and corridors, thanks to your preparations. Don't dig the obsidian! Work with what you've got. Smooth it out and engrave it if you want to. Using the rubble, build your workshops. Using the water pipe meant to feed the top of the magma block, pour a bit of water into your network and try to route it to a reservoir somehow, which can be your well or your irrigation system for your crops. Your only limitation will be wood, so make sure you chop off many blocks of wood and bring those inside to fuel your forges and make extra beds.
Once you're all done, seal the entrance with a hatch or whatever you prefer, and try to survive for as long as possible.
Bonus: Make the obsidian block in such a way that you still have access to the magma pipe or volcano, thereby giving your forges an infinite supply of fuel!