Better Modified Bass Ports are shown in Post 1 of this thread.
You have to test various mods to find out what sounds best to you. The 1 mm dimension is the height of the slots. I seal the top 3 slots and seal all but 3 mm width of the bottom slot with the modified bass port (the opening) in the center. You don't have to remove the stock internal bass vent felt. Make a cover using thin-stiff cardboard with the 1 mm x 3 mm opening centered over the bottom slot. If you make the cardboard's vertical dimension slightly larger than where it will seat, you can wedge in place, or friction mount it. Use a toothpick to apply a small bead of rubber cement around the edges to insure a good seal. Without a seal, you may end up with leakage = a larger bass port than you intended.
The best way to test different sizes of modified bass ports is by using masking tape on the outside of the vents. Cover the sides and bottom of all 4 vent slots with a strip of 3/4" masking tape. Press it down at all corners = no gaps. Even a tiny gap will skew your results. Use an X-acto knife to cut a 1 mm x 1 mm opening in one of the slots, then test. Make it larger by 1 mm increments and test as you go. I find that larger than ~ 8 mm wide is essentially = to stock vent felt fully open. Once you determine the most effective modified bass port width, you can make a permanent modified bass port as described, above, and shown in Post 1. It's also important to remember that there is a huge "bass port" around the jack in the left cup and small, but not inconsequential, "bass ports" at the cable entry points at the top of each cup. These need to be sealed with silicone or hot glue. Just be careful silicone / hot glue does not ooze into the jack - cover the jack opening with tape before you seal around it from the inside of the cup.
Dynamat / Fatmat over the ear side of the baffle, surrounding the driver, may reduce mechanical distortion. Remove the "dust cover" felt from the face of the baffle and sand it smooth before installing Dynamat / Fatmat. Roll it flat and firmly with a triple-A battery. Apply Paxmate / Silverstone over the Dynamat to prevent reflections by absorption. Cut a notch for the baffle port so it remains open. The baffle port is the 2 mm hole at the top right of the baffle. It "communicates" with the internal cup space and allows for pressure equalization in concert with the modified bass port. Note that the baffle port is also tunable if more bass is desired - see Post 1.
Good Luck
