I have just finished building my mash tun using an insulated plastic barrel. Everything works great but when I pour hot water into the barrel and put a lid on, pressure builds up inside and eventually "blows up" the lid. My question how do you deal with this? Do you let the steam slip away (make a small hole or something like that) or do you just fix the lid so it won't budge?
Make a hole. There is no need for the mash tun to be air tight.
Are you applying heat to your mash tun? If so, don't. Your mash water should only be in the 150 degree (F) range so I doubt that's producing enough outward pressure to push your lid up unless it's made from foil and even then it would just condensate on the underside.
Here's what you do... heat the water in a separate container (we refer to this as a Hot Liquor Tank) and transfer the hot water to your mash tun. NEVER heat your mash tun.