Try Large Binder Clips For a Leaky or Drafty Weber Kettle Lid
To help keep the air from leaking around the lid on your Weber kettle grill try using 3 or 4 Large binder clips (see image below). Several subscribers uses these and said they work well. I have not tried them yet.
You can also try setting setting 3 bricks on the lid. I've tried the bricks and they helped a little. Just watch you don't scratch the lid.
And you can also try removing the buildup in the groove of the lid and what's on top of the bowl where the lid makes contact. This builds up unevenly which can keep the lid from making a good seal. I use a kitchen tea or soup spoon to clean my lids groove.
There's an aftermarket gasket kit available for grills. I know nothing about them. My concern would be applying the gasket to a grill that has buildup (if the gasket will adhere properly). Gasket Video
If you use my method (minion) and setup on your Weber kettle grill and have trouble keeping the temp down at 225-250 the problem might be with the lid and or paddles (ash sweep). The lid should fit pretty tight and there needs to be play on the paddles to move but too much play can cause too much air to get in and feed the hot coals which ignites the unlit coals prematurely which climbs the temp. Keep in mind that maintaining a 225 F temp or even 250 is not easy on a kettle grill because of their size so the more air tight the better the chances.