I wanted to mod in glass barrels and bins, but agree with you all on how unbalancing that would be. Glass barrels have been used in the past: for example giant glass bottles were used to hold acids and other noxious chemicals transported to mining sites (I own some pieces of these, found at the mines.). And a good thick glass wall can be quite sturdy if done properly. But it shouldn't be nearly free, right?
The way I did it was this:
The barrel or bin requires 3 glass blocks. This is akin to needing 3 bars. Because it is a smelter reaction anyway, I just set it to do so. The reaction takes 1 fuel. each block takes 1 fuel. So unless you have magma, it's going to be as expensive fuel-wise as metal, and if you have metal you can just use magma anyway.
To address the unbalancing qualities of magma, I simply added more creatures to it. I have a flying fireball-throwing monstrosity that will fly out an attack as soon as you hit magma (or on embark if it is exposed), and a massive, slow-moving, hulking monstrosity which can walkthrough a hall of traps nearly unscathed and destroys buildings. And fire imps reproduce in my magma, so you quickly get an impsplosion up to their population limit. However, it is worth noting that in the future magma will be unstable and risky to use, as stated by Toady. Earthquakes, sudden changes in pressure causing it to flow upward, maybe implement pyroclasts and 'lava bombs' raining out from the center and destroying anything they hit as though they were a fireball...
And for those that consider sand too easy, consider the number of times an embark did not have sand, or did not have magma. Both are a luxury. Unless your a cheapskate like me and modded out all non-sand soils, but that's an entirely other matter.