I’d argue weighing with as little clothing as possible will be more accurate, since depending on the clothing that could soak up quite a lot of sweat (that you want to count as ‘fluid loss’).
Also bought a pack of these sweat patches, waiting on a hot day and good workout to use em. Doubt I’ll use multiple packs to get a more accurate measurement, since at some point you can also just get a sweat test by Precision Hydration (the labs I checked a while back were ~$100).