I managed to create my first 12 slot chest:

You are prepared to do the skill.
You forge and forge the chest construction and puuh slightly as you hammer the last bolt.
You admire your handiwork as you have constructed a gargantuan ruby vault reinforced with mithril (open).
Congratulations, for creating a 12 slot chest
you receive 100000 exp.

I even fumbled the chest creation once while building the vault, but it seems that it didn’t matter:

You take out a ruby hexagonal and start hammering it to the ruby skeleton of chest.
You bang and bang the ruby skeleton of chest but ruby refuses to bend to your will. In frustration you swing your hammer and BANG ruby skeleton of chest really hard making it shake violently. You feel like you really FUMBLED this time.
You curse everyone to hell loudly.
You laugh out loud.
You curse everyone to hell.

I lost 50+ kg of ruby in that fumble and had to collect some more before I could reinforce the chest.