Recently I needed to sort through a lot of old slides and some negatives too, because I was selecting images to scan for a book. My first inclination was to make a light box, but I quickly discovered that the cost of materials would likely be more than buying something like this <
LED Light Box, click for details>
I got the A3 size which has a lighted surface of about 16.5" x 11.7" If you need something much bigger than that, the prices go up significantly, and making one might be more attractive. The one I bought is much lighter and thinner than anything I could have made, so it is easier to store when not in use.
The color temperature of my LED light box is not daylight balanced, and is too cool, but it doesn't really matter. If you are going to scan the slides, most are going to need color corrections in post processing anyway, right? All I care about when sorting is being able to tell if it's sharp (get a Loupe 7X-10X like <
this one> - and if I have similar shots of the same subject, which one is the best one to scan. Color is not an issue until after I've scanned it.
Personally, I prefer a smooth, flat, horizontal surface so I can easily scoot the slides around while sorting. Be aware that some similar products apparently have a pattern of "dots" on the surface which you want to avoid because they will make close examination with a loupe difficult. (The one I got does not have the dot pattern.)