Double Hung Windows

They have (or used to have) slim magnets.  Getting them is a crap shoot; some phone people don't know they exist, and some shipping people can't seem to grasp the concept of "slim" (sending regular ones).  Pretty much any strong magnet will do.  Check out a better hardware store.  Or as you say, remove the dome covering.

SimpliSafe has slim magnets (see here: https://simplisafe.com/extra-magnet-entry-sensor). I emailed customer service before I ordered my system and they said they'd send the slim magnets with my order but still sent regular magnets (similar to what sevensiamesecats said). They later sent slim magnets after I emailed them again.

Yes they have them (or had them) and yes you can use other magnets of equivalent strength.

We had 7 to begin with, but are down to 2.  We lost 5 to cancers, tumors and other health issues.

I had the same issue with double hung windows and a very narrow gap between the upper and lower sash.  To start, if you're not aware, the two parts to the SimpliSafe entry sensor are a magnet (the smaller part), and the sensor itself (the bigger part).  While the magnet is pretty small, it was not small enough (since new windows have 2 pains of glass, the frame of the sash is now very narrow).  My solution was to simply rip each magnet from it's little plastic holder.  The magnet is actually about half the width of its plastic holder.  I did this with a pair of needle node pliers to grip the plastic holder, and a small screwdriver to pry apart the top and bottom half.  It took about a minute each.  I then used 3M foam tape (cut to match the magnet size) to stick the magnet to the window.  While the magnet is now exposed,  you can't see it because it's behind the sensor when you're looking at the window..  I also tested the idea of just buying new magnets to save having to rip out the ones from SimpliSafe.  It worked fine.  These guys seem to have any size/shape that you want, this looks about like the original from SimpliSafe:  https://www.kjmagnetics.com/proddetail.asp?prod=B33X0    Note that the original magnets are magnetized with a pole at each end of the bar.