I'd check with an auto salvage yard, might get lucky and find a hard cover cheap. New ones start at about $100+, but they're also on ebay. Something in a breathable fabric that also blocks UV would work too. Check with an upholstery shop, shouldn't be too expensive to have them sew one up. I found a large shop fairly close by that allows their employees to use company equipment for "side jobs", had a cover made for my custom motorcycle seat, only cost me $35.
The cover that was on my spare when I got it was so aged that I had to cut it off. Haven't decided yet on a final color/graphics scheme for the Scamp
, so I just bought a cheap one from Amazon, if it lasts until next spring I'll be happy.