The end plates provide a solid surface for compressing the cells when you tighten the all thread and a and a bracket for securing the pack to the tub. You can see the aluminum ones which are 3/16" aluminum sheet which has been cut and drilled to match the pack and has a lip that has been folded and drilled for mounting. With flat end plates you can also use aluminium angle drilled to match the bottom pack holes then drilled and screwed to the tub bottom. For pack end plates I've people use the OEM steel plates (pictured below), diamond plate, aluminum plate, one guy even used plywood... Uhm... I wouldn't recommend that.
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