Constraints in assemblies

[newcomer]

I find a number of problems when I want to create constraints in an assembly.  For example, I want part A centered left-to-right on part B.  Now I know I can get an offset by doing a mate-with-offset.  But I don't see an obvious way to do a centering operation.  I tried putting a 3D sketch in the assembly with a plane running vertically down the center, and the goal was to try to use a symmetry constraint.  But although part B had two pieces, the symmetry contraint seems to apply to two separate parts being symmetric to a plane, where I want one part to be symmetric to two planes.

I am including the file alarm.zip, which I hope has all the pieces needed to create this.  What I start with is a shelled extrusion called "case", and I want to put a mounting labeled "Adafruit 2000 mounting" which has three carefully-placed boss-style protrusions which will be threaded for three small screws.  I want this mounting centered in the case left-to-right (the narrow dimension).  Once I have that dimension established, I will place a power socket in one side, and adjust the position top-to-bottom (the long dimension) to clear that part.  Then I want to place the part "Adafruit 2000 board" on this mounting, so it does not move relative to the holes.  I can then slide the part labeled "Adafruit 1588" onto the pins.  I will be sliding this part on and off the real part; it is the programmer, which is needed only during development.  There are a couple other pieces required yet, such as a pressure sensor (it is hard to breadboard something that requires a specific operating environment that maintains a pressure environment without building the device.

Note that if the bosses, either those for the cover or those that hold the processor board, intersect slightly with the walls, this does not matter.

My plan is to make sure all the parts fit, then suppress or otherwise remove the boards, and send the case (and its cover) to my 3D printer.  I have about six such cases in the pipeline, and I want to get better and doing these.

One problem I don't understand how to handle is how to create a part whose dimension is not established until I put it in the final assembly.  For example, the mounting columns for the processor are a bit too short, because the pressure sensor, which is much smaller, will be mounted under it.  But I'd like to have the triangular-placement of the component maintained, but not have the height established until the final assembly.

Note that I have constrained the two computer components so they slide together, but always remain on the same plane.  That's OK.  I just want to make sure that there is space to insert and remove it.  Someday I may learn how to just take two parts and fit them together, but I consider that a task for another time.  My first case was designed without the attempt to dynamically insert or remove parts, and had to scrap the case and reprint it with enough space to do the insertion; I want to avoid that in this design.

I also have not been able to create the matching bosses on the cover.  I don't know what I missed, because I've done this before.

        joe