Fire Extinguisher box "hull" collision model.

Yeah so, I’ve been trying to read up on how to create a collision model for a prop so you can place something in it or go in it, but I can’t seem to figure it out. I would really need this fire extinguisher case to be one rather quickly for a project that I am doing.
So if anybody knows how to do it easily (so if it’s hard please don’t do it, asking if someone who is experienced in this subject and knows how to do it and has the time)
This is just a request if someone more experienced can make it easily.

Picture of the model (it is from BMS if I recall)

http://puu.sh/kkj4b/3b4dedf8eb.jpg

And the download for the .smd, i can give it from dropbox or mega if that’s needed.
http://puu.sh/kkj8E/bb5319265e.smd

TL:DR asking if someone can make a hull / concave collision model for fire extinquisher box.

http://puu.sh/kl5ao/d05ae576b6.SMD
There you go