I was looking for something similar, and have decided to try and make my own 'gfci cord'. The plan is to buy one of those otterbox/pelican type cases that will be big enough for the GFI cutoff (most that come from spa panels arent really breakers, only gfi cutoff devices rated for the listed load, so you need to make sure you are plugging into something that still has a overload breaker within range of your witing), wire it through with 10/4 wire (8/4 if you have a 50A setup) with liquid tight strain relief connectors. ideally you would want the GFI as close to the outlet side as possible, but your setup will likely dictate where you put the box. This way I have a waterproof, transportable GFI protected input in the case I transport my brewery.
