Alamosa, great job on this. Instead of welding the base plate, how about welding a smaller coupling, male or female, that you could place the pole in or over. This would allow you to detach it for packing up better. If you make it a bit sloppy it would also allow the base to set on uneven ground a little better.
Edit: Was just sitting on the thinking chair and thought that all you would really need to do is have a bead or small mound of weld on your plate that is smaller than the inside diameter of the conduit. This would keep the pipe from slipping out sideways and minimize the weight. Of course to go even lighter you could find a rubber end cap, like for crutches. Either to slip over the pipe or use a dowel adapter. It would probably still sink into the ground a little.