You need the filler on the left around 3" if it's a corner so that the pulls can clear each other. Be sure to measure how far the pulls extend.
I cut down my IKEA tall cab from 24" to 12" on a wall cab for it can fit my design. I also cut down several wall cabs into a T in the back to accommodate a partial soffit.
I just use a jig and made holes for pocket screws. I bought the jig for $30 from Home Depot.
I'm in the process of cutting down another 24" tall cab and make a 12" pull out broom cab.
Cut down from 24"
You can see the wall cabs cut down for the soffit. I used the extra cut pieces for the back.
It's very important to pre drill all your new holes first before using any screws or the fiberboard will crack and break around the holes because the melanine veneer is very tough.
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Ikea wall cabinets going around soffit.
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