Hello. I see you have a couple questions going about your ECB and this weekend smoke. I hope you can make the August weekend, we can talk smokers, mods and techniques till the cows come home.
As to the foil around the lid of your ECB, I would agree with Venture on this one. That smoker is designed to leak around the lid. Since you haven't done all the mods yet, Id leave the lid alone this time. The one thing you really need is to raise the charcoal off the bottom of the grill pan so that it can get air. It needs a charcoal grate. When you get around to adding the extra vents and a damper in the lid then you can seal the lid and get better heat control.
Now for the therm. Flyin by the seat of your pants? Gonna try it "old school" are ya? Trial by fire so to speak! Good for you! U.S. Marine Corps saying, " kill 'em all and let God sort 'em out!".
A good therm is something to put on the top of your must have list. I learned old school but takes alot of time and you just can't do the finesse things like getting a good roast EXACTLY rare - med. rare. The answer to your question is a bit difficult, most therms like that sense heat along the entire shaft. That means if any portion of the shaft is exposed it will also be reading the chamber temp which would give you a false reading on the IT of the meat. Most are not made to leave in the turkey while in the oven. There are some that will allow that. So which do you have? I would guess you have the one not intended for use in this way. Also each time you open the lid and put the probe in the meat you are losing heat and poking holes in the meat allowing the juices to drain away.
You say you are smoking some kinda meat this weekend. No meat therm and no chamber therm I suspect ( the therm in the lid of that ECB is WRONG ). If you can be a little more specific about your plan I may be able to offer some better advice. Good luck. Keep Smokin!