Option #1
An inexpensive light box can be purchased from your local hardware or building supply store. Look for a GE 21" x 5" Under Counter light fixture (product code 26914), and also purchase two 18" GE Sunshine bulbs (product code 13968). Remove the diffuser cover, insert the Sunshine bulbs, replace the cover, and you're ready to roll! Total cost is about $40. Here's some pictures of the fixture.
Option #2
Your local Art Supply or Camera store should have a selection of light tables from Gagne, Inc. that you can purchase for under $100. They make a line of Porta-Trace® light tables with 5000K bulbs in the following sizes:
Here is a link to a company where I purchase mine from:
10" x 12" 1 bulb List $28
10" x 12" 2 bulbs List $60
11" x 18" 1 bulb List $90
I use and recommend the 10" x 12" single bulb which lists for $28. This unit is all you need and it's the same price if not cheaper than building one yourself.
Gagne also makes a small portable unit called "Thinlite" that lists for over $100.
To find a dealer of Porta-Trace® light tables in your area, contact Kim Prentice, Customer Service, at gagne@worldnet.att.net or call her at 1-800-800-5954.
To view their line of products, go to www.gagneinc.com/products.html.