Just made this 18650 4*5 Comb Grid Holder. It can be hard to fit in second hand batteries but if they are new they will fit correctly. I recommend you trying it out and increasing the size by 2 - 5 % if necessary. You can use a raft if you need the top/bottom to be flat. I recommend the use of support for perfect bridges. 20% infill is ok but can be to low and the piece might break. Use 50% for better rigidity.
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