I was reading your post and thought "must be a 201 chassis". You didn't say it is, but I saw it at the bottom of the post, had to laugh.

There is a kind of heavy duty wire/bale at the back of the lid, on top. Squeeze each side in towards each other and the tips will disengage from the box itself and the rear of the lid will be released. It's still a pain to remove the lid though, have to pull it out over the top of that metal AC line, but it'll come out, takes some practice.

Gilly