So far I agree with everyone who said not to shield the GTO15. And I agree with the comment that if the RF is 90į to the other cables you shouldn't have too much interference
. I also agree that if there is interference it is probably coming from the actual antenna. It was mentioned a 50mm (2 inches or so) is good, but if you can get 75mm (3 inches or so) it should be a little better.
Now remember that anything that is metal near the base of the antenna (the GTO15 connected to the tuner is the base of the antenna as it is also radiating RF energy) could detune the antenna. The antenna tuner will compensate for that but there is a slight chance the compensation will be off some, thus changing the impedance at the base of the antenna.
I am sure you know this but repeating it just for taking up space here LOL. The purpose of the tuner is two fold. First, it tricks the radio
into thinking it is working into a 50Ω load. That makes the radio VERY HAPPY. Then it tricks itself into thinking the antenna is a beautiful 50Ω impedance, which we know it is not. So the tuner uses the magic of inductors and capacitors, combined with the impedance of the wire connected to the tuner, to find a very nice match for a 50Ω impedance, or as close as possible. So placing something metallic might change the second half of the tuners job to find a good match for that wire connected to tuner. But this is probably an over statement and you should be fine.
I would recommend going with distance from the other cables before using anything metallic as a shield. But, your call as you are the one who is looking at the available space.
JMHO and good luck!
If you are curious about the impedance of just the wire connected to the tuner, you can use a Vector Network Analyzer (like the NanoVNA) to check it. Nice piece of test equipment
, inexpensive, and small so it doesn't take up space in your vessel. It will also measure SWR upto 900MHz.