I use an inexpensive Oneida digital probe thermometer, the sort you leave in the meat and it sets an alarm for desired temperature or time. That model is discontinued now but it's basically like this one: https://www.amazon.com/Taylor-Digita...dp/B00004XSC5/
The gasket with a hole in it is a handy feature to leave probes in for temperature, but often I will smoke something like salmon, that doesn't work well that way, so I just use my intuition. I haven't modified my smoker at all, except for placing some wires into the three holes of the water bowl, so I can place it into or remove it from the smoker without burning myself. That seems to be the one poorly thought-out part of the design. Otherwise, it works beautifully.
I actually use a similar method to what you are doing, starting with Kingsford charcoal then feeding the occasional chunk of fruit wood on top. I have not yet tried really long projects like brisket though. I would think that stoking the coals to stay consistent for 12-16 hours could be a challenge. I think a sidecar style smoker might be better suited for those really long procedures like pork butt or brisket.
Have fun!