Winsor & Newton has many watercolour boxes. The toughest ones should be their heavyweight enamelled watercolour boxes.
These boxes are expensive because they are sold with the Artist range watercolour pans. I've seen empty boxes being sold separately but the prices are very high as well. So it's more worth while to get the box together with the colours.
The watercolour boxes are sold in a variety of sets. In this review, I'll just compare the 12 full pan set, 16 half pan set and the 24 half pan set.
12 full pan and 24 pan set use a larger box than the 16 pan set.
The half pan sets actually have dividers for each pans. That means you can only fit full pans in the middle row.
The one I'm using is the 16 pan set which measures at 14.3 by 7.3 by 2.5cm. That's just slightly bigger than the plastic Sketcher's Pocket Box. I alternate that with the half-size Bijou box when I want something even smaller.
My box has some black paint chipped off from the cover already.
They open up to a few large colour mixing wells together with another flat palette. There are openings at the corners of the flat palette which are useful when you want to let water flow out during cleaning. The large colour mixing areas are very useful.
In the space between the palettes, you can store brushes. The 16 pan box can fit a 14cm long brush. I use a collapsible brush in that place.
You can also choose to pack even more pans into the boxes. The 16 half pan set actually has space for 24 half pans. And the 24 half pan set has space for 36 half pans!
I like these boxes mainly because of the large mixing palettes. On average, it's about US$5 per half pan and it comes with a box so I would consider the overall value is quite high.
I would recommend the 16 half pan box unless you require an even larger box. If you want a much smaller box, try the Bijou box.
I've been using this box for a long time, parts of the paint are wearing off.
You should be able to find these watercolour box sets at your local art stores.
Search Jackson's Art Supplies (UK) also.