making housing for Geometrics G-856 proton sensor for underwater use

[Mtlexplr]

I have acquired a used Geometrics G-856 Proton Mag. These have been made for 31 years and the new version G-857 just adds a GPS logging function. I am looking to see if I can use this sensor underwater if it is enclosed in a properly built tow fish housing? I can machine out of plastic a housing, but don't know if anything special needs to be done? The sensor has a North arrow marked on it, so if I am running a survey, how do I compensate for the sensor facing 180 degrees from the original north facing direction when I start the first survey line?

[Boattow]

You just have to keep the north pointing arrow either pointing north or south during your survey to get maximum signal.

[Mtlexplr]

Thanks! So it sounds like if I run my survey grid east-west, with the arrow on the probe facing either north or south, that would be fine. I was under the impression the arrow on the probe always needed to be facing north. If it works just as well facing south, then that would be great! I can layout my grid then very easy on a west-east basis. Any idea how far off the bottom the probe could be for proper operation? I assume the bigger the target, the higher the probe can be, but wondered if there is a standard depth above the bottom used? Any tow fish designs available anywhere to build from scratch?