Matthew Cella shares his favorite ways to display works on metal or glass:

shelf

Place it on a small shelf
This is my favorite way to show metal pieces. Take a thin, 4” x 12” piece of wood and wrap it in white felt. Attach 2 small brackets to the bottom of the shelf and install it on the wall. Then lean the art on the shelf against the wall. Although the piece is not heavily protected, this display method enforces the physicality of the piece, and offers it for further investigation by the viewer.
All of these materials can be found at your local hardware and fabric stores.

standoffs

Mount it on a wall with Stand-Off hardware
This is the most modern way to display the art, and is found mainly in the commercial sign business. Stand-Offs are hardware that space the piece off the wall and hold it into position. There are many different kinds of Stand-Offs available, but the preferred type is one that just grips the edge of the metal, without having to drill holes.

Sources (We in no way endorse these companies or receive compensation for these referrals):

Frame Pegs:
http://www.unitedmfrs.com/cart/detail.cfm?item=4833
http://www.framepegs.com/Pricing/pricing.html

Edge-Grip Stand-Offs:
https://www.novadisplaysystems.com/basket.aspx?product=277
http://www.dkhardware.com/product-5653-eg38bs-brushed-stainless-edge-grip-for-3-8-material.html

If anyone has additional sources, please leave them in the comments!