There are a number of U.S. National Grid (USNG) technologies which work well to supplement ELM installations.  Some of them are:  

Apps

USNGapp.org - Created in 2011 by SharedGeo's Jim Klassen, this free app does one thing. It displays your current US National Grid (USNG) location using the GPS chip in your device. Once loaded, it will work with, or without, an internet connection. Location information is displayed to a precision of 33’ (10 meters). If GPS precision is less than 33’, trailing digits of the display will turn red and have a horizontal line through them. This app does not have ads, store your location, nor send your location to anyone.

Learn more and download app here.

FindMeSAR - FindMeSAR is a 'no cost' public service browser app from MappingSupport.com. Created by independent developer Joseph Elfelt of Redmond, Washington, it is intended for use on smartphones and tablets. Like the granddaddy of all USNG apps – USNGapp.org - FindMeSAR displays your phone's GPS chip location in a page of your browser. In addition to an extremely flexible mapping capability, FindMeSAR offers a slick interface which uses color-coded selectable screens so you can provide your coordinates in the format used by first responders in your area. Select the blue screen to display your location in U.S. National Grid.

Start using at https://findmesar.com/.

MyUSNG - MyUSNG reports the location of your device in US National Grid format (USNG). With a single tap on the screen, you will copy the USNG location into your clipboard. With a tap and hold, you can easily e-mail your USNG location.

MyUSNG also includes a simple map that will let you capture the USNG of locations away from you, or center the map.

The United States National Grid (USNG) is a nationally consistent language of location in a user friendly format. USNG is commonly used during Search and Rescue operations in the United States. For more information about USNG visit usngcenter.org.

MyUSNG is an Esri Labs project and not an official Esri product. MyUSNG is provided on an as-is-basis and you assume all risks associated with using this app.

Find MyUSNG at Google Play.

Online Services
 

ESRI Story Map Example: Emergency Signs on Cobb County Trails

 SharedGeo's Print Marker Online System - Zoom in, point, click, print ELM: Learn More

SharedGeo's USNG-to-Street Address Bi-direction Converter (Mashup): Use it