I just use the hobie deluxe sail bag.
http://www.murrays.com/07-3121.html I strap it to the trailer between the hulls with bungee chords.
Most people custom make their gear boxes. I've heard gear boxes commercially sold are just way overpriced. Plus, if you're just wanting to store the sails the deluxe sail bag is fine. Although, it does degrade over time due to UV. It is so big it can fit two sets of sails too. I have both my practice and race sets in one bag.
I use the bags because a gear box/sail tube isn't really removable from the trailer easily. There is no beach at my yacht club, so dipping the box/tube in the water then placing sails inside causes mold to grow on the sails. So, if you trailer/boat ramp launch your boat, be weary of that.