How bout this:
Part Number 8450-2 Gate Mounting Kit Consists of:
1 S4402-A SPDT Switch
1 M-4400 Magnet
2 S-202 Brackets
4 #6-32 x 5/16 Zinc Hex Nuts
4 #6 External Lock Washers
4 #6-32 x 3/4 Phillips Machine Screws
2 #8 x 1 Self Tap Combo Panhead Type A Screws
GRI contact for gates.
http://www.grisk.com/surface_mount/mounting kits.htm
....These ones?
There is one complaint I've had regarding these gate contacts... changing
seasons is not kind to them.
Every six months or so we get calls from whereever we have these installed
to come out and fix them. Turns out that the gates and fences move so much
that the contacts won't line up to the magnets anymore. Eventually the
fence post has dozens of extra holes in it.
Also, do not use the supplied bolts to secure anything to anything. They
will break halfway through installation.
The pull apart switch may be a good solution for you...
http://www.grisk.com/specialty/4704.htm
I haven't installed one of these yet, but have built something similar with
good success. We've had one floating around in the office for at least the
past five years (the 4704-A by the looks of it), mixed in with a random
assortment of odd contacts and switches. Looking at how it's designed
though, it looks like it would work very well. In case someone's thinking
of just unscrewing it off the post, it has that covered already. Remove the
four obvious screws and you'll find that you need to disconnect the switch
in order to remove the hidden screw. Too late though, the zone is already
open.
- Chris