I have noticed there are many Airstreamers that are asking the same question.
When New LED Marker lights are purchased, the back of the light is flush with the edge. There are either one hot wire coming from the light or Two wires (ground and Hot). This makes mounting the light next to impossible without a spacer under the light. Usually there is a groove around the outside of the bottom that will accommodate the wire. Here is what I did to modify mine so no spacer is needed.
Take a 5/16 drill and drill a relief in the bottom of the light being careful not to go too deep. You only need about 1/8" deep for the wire to clear. Move over just a little and drill another relief that will intersect the one you already did and the outside groove in the light.
This is the light as purchased
drilled relief for ground wire
You will notice I have the two wire unit and don't want to cut and push the ground into the wall. I will solder both wires and heat shrink them, then they will go in the groove around the edge of the light just fine. I am still retaining the rubber bushing on the Hot wire. These lights have a recess that is just right for them. If need be I will cut a little more relief where needed for them. I am in the process of completing this and will post pics as I mount them to show how they fit.
This shows where the light will be mounted and the bushing for the HOT wire