Custom Shelving Options?

I'd consider metal shelving just to play SAFE.
There are numerous types of woods available and also plastics, but again, why take chances. Use metal.

A free tool you can use to play with ideas, Is Google's Sketch Up http://sketchup.google.com/ I use it to bring ideas to life, my next project is building a small BMX park as a gift for my son. Hope this at least will help you with ideas.

Remember that all finished woods usually have been treated with something. Regardless of the type of wood you decide on, there is the possibility of some treated solutions used leaking gasses over time. Many types are treated for bugs, moisture, looks, etc. and if you don't know what was used, best not take chances.
Anther consideration is in case of a fire, remember that although your safe may be a bit on the fire proof side, the safe still gets hot. Anything on the inside will be effected. Any plastic will melt. Treated woods will expell gasses and/or even start to smolder.
Keeping all metal shelving would always be the safest bet.
And if you don't know how to make a shelf out of metal, check out companies in your area that may work with sheet metal such as siding companies.