Another trick... I have a little Petrol LCD hood that, when attached to the LCD in a certain (probably the "wrong way", but, hey), almost perfectly letterboxes the LCD with its elastic straps...
I've used black bristol board and precisely cut bars to fit inside the panel display then just used scotch tape to secure it. The way I measured it was, I turned on the 16:9 mode bars and then cut the peices accordingly. Then when I shot I switched it back to 4:3 mode. I like the idea of using a plastic cover so I make fine lines and still see the entire shot. Kind of like what you see in pro-production monitors.
As for framing shots, if this was the intent of your question, I taped white string on my TV set, one in the middle vertically and in the middle horizontally, then watched tonnes of widescreen movies. I started to notice a pattern where there were 3 types of framing methods, escpecially noticeable with CU and ECU shots. I noticed that my horizontal line was always either just above, dead on or just below the nose. Sometimes the nose is surprisingly dead on center of the h and v lines. I've been trying to shoot my subjects with having my horizontal line dead on the nose and when I compare them with my old footage, it looks much more professional. Hence, if I could use a plastic display with horizontal and vertial bars like a rifle scope I would love that. Although the h and v lines would have to be on a separate plastic layer so I could remove it and just use the 16:9 lines on the first layer.
Anyway, there's my long answer and I hope it helps in some way.
Remember, your footage is still 4:3, it just so happens that when youre finished, the pixels making up the top and bottom 12.5% will be black instead of part of the image. It's still 4:3 though...
I normally like to shoot my DVX in a super 35, 2.40:1 fashion using a cut out square of transparency plastic for copiers. I cut the plastic open where the picture is so I can clearly see the LCD & focus. It's hard enough with glare and everthing else on the DVX, they're not the best for focusing. Small lines can be drawn across for top & bottom lines to emphasize if needed. This lets you read the LCD screen data and see your shot while anticipating elements (actors, etc.) entering the composition from top & bottom. This is similair to the idea of 35mm eye pieces. It works great and everyone I show it to copies immediately. Use tiny bits of gaffer tape to attach the four corners.
Too bad Panasonic doesn't give an electronic 1.85. and a 2.40, and give us the 1.78 with top & bottom lines beside the bars. I like to look outside the frame because it really makes a difference when framing.
Oh if this seems corny, this is how the 35mm film setup works. They shoot academy 1.37, but frame for the aspect they want using the eye piece and then the theatres mask the image with these little metal plates carved to 1.85 or 2.40. If they took those plates off the image would be all over the audience.
In post I use Avids bars for keeping aspect. It lets me shift the frame around in post just in case there's a better frame available. It works great. Just like they do for 35 mm.
From what I've observed and learned it's that simple.