All these are from memory. Hopefully someone will pop in soon and tell us all how bad that is...
Outside diameter, "5" is about 1", give or take some thousandths. 1" round stock is close enough.
Hole "3" (top to bottom) is just over 1/4" for a 1/4-20 bolt (cap isn't threaded)
Holes "4" are for the 8-32 side screws.
Holes "3" and "4" centered on the original back cap which is (from memory) 3/4" thick, they are also centered on each other and exactly perpendicular.
Distances 1 and 2 are about 3/8" but as long as the back of the hammer doesn't hit the cap, it's good. There's wiggle room.
Distance 6, being centered on the cap, would be about 1/2".
The way i made these things was to just field strip the marker but leave the hammer in.
Pull the hammer back and put it on "safety" and measure the distance from the back of the hammer. That's your max depth. Subtract a little for good luck.
Now stick a chunk of 1" round stock in there to your measurement and mark it. Mark the side-to-side back holes (for the two small screws) and drill, preferably in a drill press. Don't bother cutting the round stock to length yet.
Tap the holes and screw it in place.
Mark for the top hole. I usually cheated and did this by putting the whole marker in the drill press and just drilling it, being careful to not quite go all the way through (so as to not cut the threads in the marker itself), then finish the hole after removing the cap.
After the holes are drilled and tapped, mount the cap in place so you have your marker with a length of round stock sticking out the back.
Lastly, line up your stock to the bar stock and figure were you need to cut it to get it all to line up right.
Two wrongs is a u-turn. It takes THREE wrongs to make a right.