The Seattle Audubon Nature Shop (and other places, I'm sure) sells suet
feeders in which the suet holder is enclosed in a larger cage. This
design has worked very well for me. Small birds fit easily through the
cage mesh, but squirrels and bigger birds can't get in. Flickers can
stick their heads in far enough to get the suet with their long tongues.
The starlings in my yard used to be able to get at the suet by perching
on top of the cage, but I put a dome over it, which keeps them off, too.

Rachel Lawson
Seattle