Go nuts: http://www.reallyusefulproducts.co.uk/usa/

Put a zip tie through each end of the top and box lip to secure it. Nearly all their boxes are the same design, regardless of size. Anything from thimble-sized to trunk sized.

This is a typical lockable network product enclosure: http://www.youtube.com/watch?v=DWCfffFZs_k

If you have a label printer, you can label this one "The Internet - Do not disconnect!"

http://www.eplastics.com/Plastic/Display_Cases/TLLB10X10X10BLK


Lots of hits searching for "lockable plexiglass enclosure" or "lockable plastic box"