You could build the inner frame thinner but that reduces the amount of space for insulation.
Build wood front door.
Cut 2 pieces of 2 by 4 lumber to length for the sides of the door known as stiles.
There are a few tips for dealing with oversize parts breaking assembly into manageable steps fitting the door and installing the hardware.
Cut them 1 4 inch shorter than the inside height of your door jamb using a miter saw.
I recommend an inner frame made of wood that measures a full 1 inch thick.
How to build a door step 1 the inner frame the exterior face of an insulated door built using the design shown in the plans.
And you can build a much better door than you can buy for the same money.
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Once you master those you re on the way to making your front entryway into a showpiece.