Well I don't have any boxes handy, but I have a tray I fabed for a custom dash. It is approx 6"x4"x2"
Here you can see the inside corners are rounded,
and some air bubbles, but no big deal, they sand right out.
In liquid form, it will run into ever crevice, here you can see the grain from the mold.
Get some cheap 1/8" paneling that has a smooth side and something you can glue together with regular wood glue. Use it to create your outer shell. Don't worry about rounded corners on the exterior at this point, make them all squared, you can use a file or sand paper to round it off after words.
Next create the inside chuck, I use pine wooden blocks or you can use balsa blocks also. Shape your inside chuck just how you want it, rounded corners and everything. Remember to make the chuck allowing for wall thickness.
The tricky part is to keep the chuck from dropping to far into the shell, making your bottom wall too thin. So, attach the top of the chuck to a plate that is larger than your outer shell. You can also attach centering guides to this plate so the chuck stays the same distance from the interior walls of the shell. Also, because the check is attached to the plate, it can only be lowered into the shell so far.
Here is a drawing of a box a did a while ago, it shows the top view of the shell with the chuck inside it, the top view of the chuck, and the resulting box shape. Later I can round the exterior corners.
Here is a side view drawing of the chuck being lowered into the shell with the center jig and guide rails.
Once you are ready, mix your plastic and pour it into the shell so it fill about half way, then force your chuck into the shell allowing the plastic to ooz out the top. This also forces most of the air out.
Note: Make sure you use the Smooth-on sealer and mold release as directed!!! or your plastic will stick to the material. A trick I use is to cover the entire chuck and shell with clear packing tape. This makes a nice smooth surface, seals the chuck, and acts as a mold release, as the plastic won't stick to the tape. It does however leave a mold line where the Tape edges come together, but those are easy to sand out.
If I get the time, and anyone is interested, I will try to do a step by step procces with pictures and post it.