Hello, and welcome to the Discussion Groups!
So, what you are looking is basically a generic box with adjustable Length/Width/Thickness? This is pretty easy to whip up:
⚀ Start a new Family using the "Electrical Equipment.rft" or "Electrical Fixture.rft" Family Template.
⚁ Create an Extrusion using the command on the Ribbon, and draw a square (any size) around the origin.
⚂ Create a new Linear Dimension ("DI") and click on the bottom line, the top line, and then the horizontal origin Plane. Place the dimension somewhere to the side of the lines, select it, and click the "EQ" to equally constrain them.
⚃ Create another Linear Dimension and click the bottom line and then the top line, and place it off to the side. Select that dimension, and you'll see a pulldown named "Label:" on the Options Bar (below the Ribbon). Click that and select "<add parameter...>", and name you Parameter "Length" and make it an Instance Parameter.
⚄ Repeat steps 3 & 4 on the other axis to create the "Width" Parameter and constraints. Click "Finish", select the Extrusion, and under Properties, click on the grey button next to "Extrusion End". In the following dialog, click "Add parameter..." and create the "Thickness" Parameter just like the other two.
⚅ Finally, under "Family Category and Parameters" on the Ribbon,enable the option "Work Plane-Based" and disable "Always Vertical" and then this Family will be placed and hosted to surfaces in the Project.
I post this because it is far better to start learning some Family creation than to just be handed a Family. If you have any questions, don't hesitate to ask!