then what you need is some small strips of cedar to creat a seal for the box you want to convert. on the box you have i would put the lip on the bottom section it would be easier to measure and attach for a novice wood worker. you have dividers from the coffins and if you want shelves your can make youself a lilttle one that sits on blocks. could be done fairly easily with just some glue and a little time. pretty low budget too.
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