There are 2 spring loaded arms on the sides to which the cover is to be attached to.

aprilneverends - I am open to suggestions. I've spent hours and hours looking. The lights aren't that close together so a little difference is OK. I just want to be sure the light fixture isn't real dark. I am trying to find that out and might need to call the company. Donna - at this point I don't want to do more electrical and have to patch drywall. Otherwise that would make life much easier.

You could try these. https://www.platt.com/platt-electric-supply/Boxes-Round-Open-Base-Covers/Wiremold/V5737/product.aspx?zpid=61446 The 4.75" diameter might let you slip the base right over the extension. The 5.5" diameter would leave a rounded edge all the way around the fixture base. The extension could be painted the same color as the wall, or there are metallic color spray paints that might match close enough.

The voltage "disappears" because your meter is reading junk. Its impedance is so high that it can read "phantom voltage," even when the circuit's resistance is so high that no usable current can flow. When you put a real load such as a light bulb on the circuit, the truth is revealed. :) A low impedance tester (example) will give you more realistic results. For a cheaper low impedance tester, get a rubber pigtail socket and a small (7-25 watt) INCANDESCENT bulb at the hardware store. If you have a soldering iron or gun, tin (coat with solder) the wire ends to stiffen them so they're more like tester probes. In your case, the circuit is open somewhere. A failed switch is a strong possibility. Use your low impedance tester to check both switch terminals (put the white wire on the metal box, or on a bare ground wire in the box, and touch the black wire to each switch terminal in turn). Careful when working with live circuits! With the switch on, both terminals should light the bulb. If not, replace the switch. If you get real voltage at both switch terminals, but not at the black wire in the light's box, post back and we'll dig deeper. The link is for illustration, and is not an endorsement of the product or vendor.