A few months back Serious Reefs made a video on how to make your own acclimation box by getting a plastics/acrylics company to cut you specific size pieces and then you glue it together yourself. I followed the measurements they had for a 180 gallon tank (which is what I have) and it was way too big. I built a box that was basically 12" x 12" x 12" and it cost $40 for someone to cut the acrylic.
I am planning on reducing the measurements a bit and trying again. You have to subscribe to their Patreon to see the video, although free Patreon trials seem to be common.