The outer edge of the meter's plastic bezel is transparent and can facilitate easy side-lighting of the meters. As a "bonus", the plastic has "prismatic" finish around the edge and acts as a light diffuser, resulting in a more even illumination of the face. I attached two yellow-orange high-intensity LEDs (clear lens, from RS) with hot melt glue in the middle, right under each meter (~1/4" away from the bezel - this widens the beam and covers more area). It might take some experimenting with the position of the LEDs in order to achieve best results. This is actually the most difficult part as the space is very tight to get the glue gun in and to fix the LEDs in the right position. The LEDs can be any color - i just like the warm orange glow - it reminds me of the long gone tube radio era.
Two current limiting resistors (470 ohms) and a filtering capacitor (100 uF/10V, to avoid flickering as the 5V DC in the LED circuit is rectified by a single diode) were inserted in heat-shrink tubing and then attached with hot melt glue to the bracket holding the meters in place.
The LEDs are powered by the same circuit, powering the front panel "ON" LED. There is a second winding in the power transformer, used to supply the regulator board with 5.5V (hence the 0 to 15.5V adjustment range). The AC is rectified by a couple of diodes - one for the voltage regulator circuit and one for the front panel LED. This picture shows where my illumination circuit is connected on the regulator board - the positive lead is taken from the "hot" side of the front panel LED current limiting resistor (just after the diode V5) and the negative lead is on the 3rd pin (from the right, looking at the front panel) of the connector (black wire).
This is how the meters look in a well lit room...
... and in the darkness of my ham shack.