Consider a cranked (necked) bolt next time, maybe?
It's MDF...the worst material known to mankind. You'll need chisels like a razor to deal with it, and scoring does no harm.
John
Your chisels weren't sharp enough - you can see than from how fuzzy the top and bottom of the recess is. A Stanley knife shouldn't be necessary if your chisels are properly sharpened. I can use my chisels as a marking knife because I keep them sharp.
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