This is a simple remix with a slight redesign to the blade slot making it easier to push the blade in without breaking the outer plastic part. I broke six of these before the redesign and now have not broken one since.
Simply hold the blade, carefully, with a pliers feed into the slot in the direction as shown in the image and push down. Once partly inserted I find it easier to place the nose of the pliers in the slot in the blade, push down as far as possible and then again grab the blade with the pliers, grip tightly, and push all the way in until the tip of the blade sits inside the slot in the plastic wall.
This is specifically designed to hold easily found surgical blades, code 10A as shown in image, available from the likes of eBay and Amazon.
Ensure your printer is well dialled in and not over extruding, otherwise the blade slot will not be clear.