Have you looked at the stitching flat scanned images tutorial for Hugin?

Summarizing the tutorial:

• set the horizontal field of view to 10 degrees with crop factor = 1.0x
• assign each image a different lens number
• create control points as you usually do (manually or automatically)
• optimize roll, x, y and z for all images except your anchor image (do not optimize any of the other parameters)

This should give you a decent (but probably not great) stitch. But for your purpose it will be good enough, I think. This is as far as the tutorial above goes.

Now it is time to render your pano. You need to render aligned individual images, that you will then compose in an image editor, since Hugin cannot add a border and drop shadow effect to each image. To render individual images set the "exposure corrected, low dynamic range" option under "Remapped Images". Then in the "Processing|Remapper" section open the options for "Nona" and make sure the "Save Cropped Images" option is not checked.

Using GIMP, load your individual images as layers into a single image and sort the layers to your liking. Then for each layer select the layer and run "Filters|Light and Shadow|Drop Shadow".

Finally, add a background layer with the background color you like, and export a flattened image!