Daniele Fua :

what I need is just a simple, cheap and weatherproof repeater at the helm

I am affraid the weatherproofness and the general robustness are hard to
achieve, and you will be better off buying a moderately expensive commercial
product.

I am a big fan of the TackTick products: you just plug inside the boat an
nmea receiver that forwards over wifi the data it receives, and then you can
hang as many wireless repeaters as you like in the cockpit. The repeaters
are entirely wireless, using solar power (they have enough battery to run
through the night), and wifi to communicate with the base.