I think you misunderstood how I did this.
The tank already being drilled actually helps you. If you have a custom weir box made it will not be drilled for the bulkheads. As such you can line it up to be flush with the top of the tank like how mine is so the weir slots are inline with the trim. Just be sure to measure the height from the underside of the trim to the bottom of the current weir box you have. The new weir box needs to be 2.5" deep so the bulkheads will fit in it.
Sent Modular Marine a message. Hope to hear from them soon. I just hate spending money that could be on some else now.