Mine has the same problem. It seems to sag in the middle.
In this photo, you can sort of see the sag. Well OK maybe you can't.

Here is where I have pulled down the gasket slightly. I was thinking of puting rubber cement behind the gasket to help "keep it up". It was also very nasty & moldy behind there due to condensation & what not. The unit was used when I got it & it came that way. I cleaned it out with spray cleaner.

I figured that before glueing it I would try a piece of weatherstripping. I used some of this.

And did this.

I have not given it the flashlight test because I put the unit back in service right after this mod. I think it is holding the cold in a lot better then before because my on/off cycle times have extended.
The weatherstrip is a Frost King R538. You can get it at OSH, Home depot etc. It comes in other sizes. Just make sure you use a closed cell foam. It comes in 10' lenghths. You only need 24". If you want to try this mod send me your address & I will mail you 2 feet.