How do you build a portable lightweight frame for trees?
PVC pipe might be a fairly cheap option.  Though exposure to UV will reduce its impact resistance over time, so you might have to replace parts over the years. 

If you dry fit the fittings, you can assemble/disassemble as needed. Which would reduce the space required for storage, and increase portability. 

You might have to use sandbags or stakes to keep them in place, if it's windy in your area.