What material can be used to bond metal to metal and is removable in the future?
From your description, I'm visualizing that you want to attach a cover (metal to metal).  If the parts mate well, superglue (cyanoacrylate) may do what you want.  


The original thin liquid type works best when the surfaces make good contact so that there is a very thin layer of adhesive between them.  Such a joint will be close to airtight.  It doesn't work well as a filler. 

There are several types of superglue gels.  One type typically has fumed silica as a filler, which will join poorer-fitting pieces by filling small gaps (but they still need to mate well).  This may be a little harder to separate, if needed.  Another type of gel uses a material like rubber as a filler to also make it more shock resistant.  That will be much harder to separate, so you probably don't want to use that.
The glue has great strength for keeping pieces from being pulled apart, but very low sheer strength.  When you need to separate them, you can sheer the joint by tapping on one of the pieces in the direction that would slide it.
It works well when mating two surfaces of the same material so that they expand and contract at the same rate.  With dissimilar materials subject to temperature changes, the expansion of one relative to the other will sheer the connection.  
It dissolves in common solvents like acetone.  You can also separate the parts by letting acetone wick into the joint, and you can use acetone to clean the parts for reuse if you need to separate them.
40 F to 120 F is well within its service temperature range.