JW411 has nailed the danger. Avgas in liquid form is not such a problem... it's the gaseous form (read fumes) that is the real danger. Ensure there is not the slightest possibility of leakage via puncturing, or leakage from the cap.
A cabin full of Avgas fumes is a bomb ready to go off at any incidence of a spark... let alone the extreme danger of breathing in the fumes.
Remember that volatile petroleum fuels also have a very high expansion and contraction rate with severe temperature fluctuations, which can create a danger of the container bursting, if no allowance for expansion is made.
Even steel jerrycans aren't immune to splitting and leaking under pressure. Keeping the containers upright is a good move, it helps keep pressure off the cap seal, and will help reduce spillage in the case of leakage from the cap.