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1-- change the battery box if that is not dificcult for you , and you sure the final wood work will be Good, this EQ are prepared for 18V or 2--if you newbie in working on wood, maybe is better not to try to open space for a bigger battery box, use just one battery, this EQ board also good work with 9v, they are prepared for 9v also or 3-- if you really want 18v without open a hole in the wood , easy way is to put 2 battery , i mean, 1 battery in the Battery box, and the second near to the the EQ board ,offcourse if have space there to put the battery ( you have to put some king of isolation between the battery and the Eq circuit.)