I also have a big homemade smoker with a big door. I spritz mine halfway through the 3 hour section. I give it 90 minutes and get all my stuffage ready, open the door, move the ribs just a tad to insure they are not sticking, and SQUIRT SQUIRT SQUIRT, then close the door quickly. Total time open is usually less than a minute, maybe 90 seconds if I am taking pictures for folks.
Same thing when I am foiling. I have pans laid out with the foil already lined on the table. I open the door, grab the ribs with tongs and place them into the pan. Grab the next rack and put them in their pan and so on depending on how many I am cooking. Once I get the ribs off the grill and into pans, I close the doors back up and handle my wrapping and juicing. Once I have all the foil pouches ready, I open the door and place the pouches back inside quickly and close the door. Total time with doors open 60-90 seconds.
Another trick I use sometimes if I know the door will be open for a while, I get a chimney of lump started off to the side and add it in when I close the doors back. This gives a quick burst of heat to get temps back up to 225ish. It just takes practice to learn how much fuel to use to regulate your temps.