Suet feeders are made of some kind of wire screening allowing the suet cake to be set inside the container while making it available to birds by sticking their beaks inside the wire mesh.  

The simplest and perfectly acceptable efficiency are the wire cages sold at garden centers, bird feed stores, home centers, and even hardware stores.  

Fancier cages are sold on websites such as www.duncraft.com and www.audubonworkshop.com. 

For information about suet cakes, commercial and home made, click here.