[mental4astro (Alexander)]

2mm thick steel for a box is one heck of a box!

You can use your stick welder. Thin stick with the right power setting. Start with two or three spot welds to join the components together, and you can then do strip length. If buckling is a concern, you can do a series of spot welds, or beads first. This way the parent metal isn't subjected to a huge heat surge when it can buckle.

For sure practice first. Old steel bed frames are not only good to practice with (cut up sections), but are also great for making frames for stuff in the backyard. I've made several stands for bbqs (3 of them, one being to replace the crappy one the bbq came with), for a fire pit made out of the stainless steel drum from a clothes drier, and a stand for a wood oven. Treat the steel appropriately afterwards and it lasts for years.