Hmm... that would make it quite versatile.
The bottoms can be either left flat, or have grooves milled into them so that they stack. With the regular 39mm trays I groove the bottoms to match the tops so that they can stack. I guess I could groove 5 rows on the underside of the 4 row rack so that they stack onto regular 39mm trays but it might be a tricky fit.
Example of my regular 39mm racks:
View attachment 93638 View attachment 93639
I suppose I could offer different options, but that would increase costs since I can no longer batch the work together. I'm in the process of making a few dozen racks but before I got too much work done, I wanted to see if there's demand for 44mm trays.
The flat bottoms work well but then the trays slide around when stacked. It's easier for me, not complaining but I always try to keep functionality in mind.