Pretty cool sketches. Looks like it could work.
I tend to go for pre-existing items for pole bottoms, however. Nylon barbed plumbing fittings and plastic champagne corks come to mind.
With the downward pressure of the tarp and guylines I haven't had a problem with the poles moving laterally at the bottom. So I'm not sure the protruding bottom on your drawing is needed. The shape of the bottom would also tend to hold on to dirt and mud.