If you cannot find something suitable as a replacement, you might consider going to a home alarm shop. Most of the window/door perimeter switches are nothing more than the reed switches you described, and they come in a variety of packages and sizes. One in particular is about the size of a pencil, encased in plastic, and made to mount end to end with the magnet which is drilled and pushed into a hole in the adjacent door surface to the reed switch. Keep in mind there are two types of switches sold, normally open and normally closed.
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