Sorry for the late reply, the last few days have not gone smooth as we have been updating our computers and I lost a lot of time with some unforeseen incompatibility issues.
I have checked this out and the included sets are 6025.515, so, you could upgrade by adding 6025.500, just split one set as 6025.515 is half a magnet like the 6025.500 (neodymium) and half of a weaker ferrite magnet. They didn't make this easy as the ferrite magnet is premounted on the box itself and will have to be very carefully removed by unhooking the mounting tabs without breaking them. I am supposing they did it this way because the ferrite is absolutely inert where the neodymium is coated to waterproof it and it is a remote chance the waterproofing could be breached. This should bring the hold up to 3/4", this would be the upper limit. I also checked an even stronger mount. You could do the same with 6205.500, move the existing stronger magnets for the outside onto the inside and use the 6205.500 on the outside, theoretically the hold should then be about 7/8".