Lately I’ve been firing in the 2.5″ tall box rather than the 4.5″ tall. I elevate it on posts (broken tiles will do) so it’s as high as possible in the kiln chamber. The bottom of the box is right above the lowest heating element, so the heat can flow under the box and up.
I use this box in both the front loader and the top loader. The size of the box and the placement in the kiln allow me to fire in two layers in the front loader as well, with full sintering!
I also stopped using the lid, so any stainless steell tray that is no higher than 2.5″ will serve the purpose. My student Suzanne Roady was unable to sinter pieces that kids made in a school project. The pieces were 0.5″ thick. Elevating the box and removing the lid resulted in full sintering.
Firing without a lid, though, will require purchasing carbon more often. A lot of it burns leaving ashes behind.