Base: 6mm Front and 3mm Sides
Our default acrylic thickness configuration is 6mm fronts and 3mm for everything else.
The reason we opted for these is it works quite well and minimizes the price. The door gets the most use and thus the extra thickness. The top can support the average hobbyist using 1Kg filament spools, no problem.
We typically use 5Kg spools, however, and under repeated 24/7 use the extra weight can develop cracks, so some customers prefer to upgrade to 6mm tops. This is also helpful if you want to store an electronic filament dryer on top, or a school or library where kids have tried to climb on top.
6mm Front and Sides
The all 6mm configuration has been added to some models as a request from customers for whom price is no object. Since the majority of heat loss is through the sides and top, this increases the internal temperature by a few degrees. It is also sturdier, which only makes a difference if you intend to frequently move the enclosure.