sorry to get a little off topic, but I'm curious, greenhouses are notoriously humid (unless they're empty). why not just air dry outside? how much seaweed are you talking about?
I doubt you'll be able to find space in late April regardless. maybe in the heat of summer or dead of winter, but not the week or two around last frost date..
or let me rephrase. you wont find one on a garden forum, lol. we're all itching to get started already.
maybe post in craigslist or facebook marketplace? I think your only hope is to find someone who's NOT a gardener.