Those sure are some oxy looking things. I know you just copied what the store bought ones look like, but they don't have to be so big or shaped that way. Just make a box a little bigger than your hole, maybe 10"x10" inside or 12"x12". Maybe 8"-10" high or so. Make a removable lid with a handle, you can do this by putting little blocks in the corners of the box to hold the lid, they can also help hold the sides together. Then put those little white plastic clips on the outside to hold the lid on for transport, the kind you just turn. In one corner put a single deep metal electrical box to hold your charcoal. Put your spool in the corner across from it, just make a metal bracket and put the spool on a small machine screw. I just use regular black tipup spools. Just make sure it spins freely. If you want tension on the spool, use a lock nut. Then you have everything but the flag.
To make the flag, use a piece of wire the size of a hanger wire. On one end weld a flat piece of metal in the shape of what you want your flag to look like. Some guys use letters, like the first letter of their last name. I just have a diamond on mine. Then slide a weight onto the wire, it can just be a big bell sinker. On the other end of the wire, bend the wire to make a slight hook. Just before the hook, you will need a stopper to keep the bell sinker on the flag. Just wrap electrical tape around it until it's thick enough to stop the flag. Cut a slot into the side of the box between the spool and charcoal holder. The flag will go in this, try to make it so the hook part of the flag reaches the center of the box (your hole). The part on the outside should be the same length as what is inside the box. Attach the flag on the inside so it can teeter up and down. You'll need to cut a slot into the lid as well.
Now use a split ring to mark your depth. Put the ring on the hook end of the flag. This hooked end should not be much of a hook though, you want the ring to slip off when a fish hits. Just enough to hold the ring in place. Use the bell sinker to slide up and down the flag for sensitivity.
With this setup, I can see fish mouth the bait. Crappies will bob the flag when it's set very sensitive, usually it's tripped by the time you get to it. Plus you can hear the flag trip as the weight hits the side of the box. They essentially work like a tip down in a box.
I also have rectangular shaped boxes with flip lids, but the flag on those is a little different in that it's shaped more like a long 'S' and goes through a hole in the side of the box as opposed to a slot. Works the same way though.