Since you are using tubes for the 'frame', I would weld round BAR stock for the braces... depending on the loads, they could be 1/4" or larger. Because the loads are small, it's easy to develop enough weld to resist the forces, also welding of the face of the tube is not likely critical. Be careful of interference where the X-bracing crosses the opposite bracing.
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik