Sorry sir you're stuck for safety reasons with a switch on the NEG side of the battery.
The cranking voltage is 12, it's the 90-100 amps CURRENT on that diesel starter that you have to worry about.
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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.
Since it's to prevent discharge of the main battery whilst sitting unused for long periods, next to the battery should be fine in which case it could arguably be in the +12V side.
However, it should be on the GND side as Howie wrote - ESPECIALLY if remote from the battery. (Which battery terminal do you disconnect first & reconnect last? (And why?))
For mere isolation purposes, any suitable mechanical "battery isolation switch" should do - ie, a mere single-pole switch. (These should be rated for ~200A or higher or 400A peak to handle high cranking currents which can be well over 200A tho reduction starters often take less.)
I advise against any relay implementation. How do you ensure the relay does not open or fail when driving?
[ Granted, I know of local competitive vehicles that have such implementations, but they have been passed by Scrutineers with even less expertise & intelligence than their Regulators! ]
I did write about hotside versus GND isolation in threads like
newbie problem, engine kill switch (page 1) which included a "corrected" version of published the
FIA Approved Isolate & Kill Switch. (Despite their recognition that more than a single pole is required to isolate the battery AND kill the engine, for some stupid reason the battery isolator is shown on the hot side (+12V) which IMO should obviously be understood as being hazardous - especially when these switches are required at rear corners of front-battery vehicles! But it's not the first time I have seen stupid ideas and implementations by Regulators.)