ebay..
I picked up a “shop-soiled” one from a volvo dealer.
Branded as a “Volvo weekender”, is made by Thule, so is the same shape as one of theirs, but is a more flexible and lighter plastic, so easier to lift around, and more durable to impacts.
Similar to you, it gets used for summer and xmas holidays. I keep it in the cellar at work as we’ve no space at home. I do keep the roofbars (also bought off ebay from a ford dealer) at home in the loft though.
Box cost me £80, bars cost me £60. Cheaper than buying and running a bigger car!