That chest mount is a urethane housing so no need to worry about ss on that. A couple of things to consider-
If you do the ss175L & ss175HW as mentioned before you can run both frequencies at the same time on the different screens, not sure you can run both at the same time with the chest mount you have picked out. Not that you will always want or need to run both frequencies at the same time but maybe sometimes?
Paying for fabrication i would think offsets the cost of the 2nd transducer
Just my 2 cents
You might as well start looking at the 2-3 KW stuff, what the hell!