You use the word - SLIGHT - when referring to the curve on the mounting surface.
I would measure the height of the curve and then take a look at the object that you want to mount on that surface.
For myself, if the difference was only 1/16th or so, I would use a sander or grinder on the objects mounting face and grind material off of the object to be mounted so that the object mounted perfectly on the surface.
For this particular item, I am making an assumption that it is solid metal and not hollow.
If the item was hollow, of course I would then use a shim as has been suggested by others.
If you do have a solid object and it would make sense to have the base of the object changed to match the contour of the mounting surface, but do not have the tools to do the shaping, then - swing by a machine shop and have them mill the part to perfection before mounting.