I made an R2D2 costume for my daugher last Halloween. It was made out of dollar store foamcore and posterboard. The dome was made out of a series of strips, some of them being 2 pieces of posterboard taped together, the whole dome was put together with masking tape and had a foamcore frame to keep it from getting crushed, the dome was finished with silver spray paint with paper details glued on with glue stick.
The body is a foamcore frame wrapped with a posterboard skin also with details printed on paper and attached with gluestick.
Same construction on the legs, but I ran out of time so there are almost no details

You can see the hot glue gun on the counter just before we went out trick-or-treating
If you would like some interior pictures, I can dig the costume out of storage
FWIW: i used the little R2 as a starting point for my project. but ended up drafting new parts using Qcad