I use the excel method on white decal paper. I find that MS clear backed numbers look terrible unless you paint the surface white. That's great for locos with clear inserts, but I have made my own on resin models or older non lit models with inserts (like original Kato SD40-2).
Paint the inside surface black, then fill the space with Krystal Klear, paint a tiny bit around the edges black so there is no light bleed, then apply the decal.
Turns out great and you never have to apply 16 tiny numbers in perfect alignment again!!!
Craig
Edit:
And when the decal is dry apply a bit of Kristal Klear on top as smoothly as you can. Once it sets but is not fully cured, use the shiny side of a flat piece of styrene to stamp it so it's perfectly flat. Gives you a bit of the effect of the glass panel on the front.