In case it helps anyone I just bought 2 custom dosing containers from advanced acrylics. They had the best price and very happy with how them came out. I’m in Pa so was worried about the shipping but they were packed great and not a scratch
Check out Midwest Custom Aquariums. Got mine in 2019 with external overflow with no center bracing (euro bracing). They recommended thicker panels and seem very knowledgeable with structural nuances. They’ll help with specs based on the design and operational details such as return pump flow.