I have never cared for top boxes, preferring a combination of different bags selected for a particular trip. This system provides much more versatility, luggage volume, and certainly weighs much less. After going through an assortment of military, marine, and MC bags, duffles and stuff sacks over the years I tried the H2W bags noted in a previous post (helen2wheels.com).
The bags are top quality, waterproof, durable, and have strap loops all over which are suitable for any kind of tie-down situation. I also use her "Super Packing System" tie-down straps for everything - very simple and secure, just D-rings and nylon strap. If you try her straps make sure to get a pack of strap extensions just in case. The D-ring end of her standard strap is short and can be used for most but not all tie-down anchor points.
I started with one of her small clothes sacks that are easy to get in and out of and to find things. I now have an assortment of her clothes sacks and stuff sacks and am completely satisfied with both. As noted in a previous post, I use the stuff sacks to pack things I/we won't need until we get to where we are going.