The a/c units keep getting smaller, when unit replacement is slightly smaller , the extra space can be filled with a piece of plywood or rigid foam insulation with a thin layer of plywood, any edges seen from inside can be covered with trim, make sure the wood covers entire bottom layer of sleeve to properly support. or just find the larger unit if you can. hope this helps.
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The a/c units keep getting smaller, when unit replacement is slightly smaller , the extra space can be filled with a piece of plywood or rigid foam insulation with a thin layer of plywood, any edges seen from inside can be covered with trim, make sure the wood covers entire bottom layer of sleeve to properly support. or just find the larger unit if you can. hope this helps.