That is a low slope yeah, but for a full standing adult it's workable. Where is the control panel going to go? Are you planning to attached MDF/MDO/Plywood to that outside? I worried about load and stability. The way with which you've built the skeleton leaves me with many questions. The metal pieces you have for reinforcement won't hold weight unless you use Bolts and then the wood you used it a bit thin for that depending on what the cab is going to be holding. Do you have any sketchup drawings of the complete skeleton plus outershell?
I guess what I'm saying is while the slope is low, I'm far more concerned with the skeleton infrastructure holding weight than I am about the slope.