I would recommend
THIS ON-ON SPDT SWITCH for the job. You just need to install it in the middle of the jumper, i.e. cut the jumper, solder one end to the middle lug of the switch and the other end to one of the outside lugs. Doesn't matter which one--just leave the other outer lug unconnected and it will then function as an on-off switch.
Something you need to consider is where the switch will be mounted. I would look inside the DS-1 enclosure and see where there's enough space to mount this small switch through a hole you will need to drill in the enclosure. Shouldn't be hard--these mini-toggles don't take up much room at all. You can come out through the top of the enclosure or through one of the sides, depending on where the switch fits best. Just be sure that the lugs don't contact any other metal/conductive surfaces. I'd also suggest that you remove the jumper from the back side of the PCB and install it between the legs of the jumpered components on the component side of the PCB, since that will give you much more convenient access to wire it to the switch lugs.
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