I have a old tent skin that I use to cover my trike if it's outside and I use a couple of old guyropes either pegged down eiither side of the trike and running over to hold the tarp down or if I can't get pegs into the ground I wrap the guylines around to hold the tarp on.
I also carry an old waterproof rucksack rain cover all the time as it slips over my seat back and keeps the seat nice and dry