The staff was extremely helpful in finding a lens cover that would work for my 24" undercabinet light. She asked for photos, and when that wasn't enough she asked for a couple more with specific instructions of what to photograph.
After that she came up with their best estimate of what variant of their hundreds of covers would work. Height and Depth were perfect, but I did have to take about a 1/2" off the width. Not an issue, just used a hack saw and it fit perfectly.