Ribosome is a nice, simple program for constructing a structure from a sequence.
It is free and can be obtained from http://roselab.jhu.edu/~raj/Manuals/ribosome.html and installed quite easily.
Once installed, it is useful to set the Ribo environment variable:
>export Ribo=/usr/local/Ribosome # or wherever it happens to be
and include the src directory (or wherever you put the executable) in your path:
The documentation can be found at the above web site and at $Ribo/doc/ribosome.html