We have the Joovy Ultralight and didn't bother with the official cover because it looked like it wouldn't fit when using the car seat adapter. I just had a look at tandems with similar dimensions and ended up just buying a generic raincover from BabyTravel Ltd through Amazon to fit a Graco Stadium Duo. Its a wee bit big when the car seat isn't on but still a decent fit. Is the Big Caboose the one with 2 seats plus the bench seat at the back? Don't think the Stadium Duo cover would fit over that (maybe without car seats mind you). Might be worth trying to find out the longest tandem on the market here and see if the raincover for that would fit.
Sorry if that doesn't help at all, it just seemed expensive and a faff to me to ship over the Joovy raincover so thought I'd make the suggestion.
Good luck