[H4]

Will it be under water or just exposed to rain? I have yet to see an aftermarket box that was truly water tight. You can get military surplus ammo cans in different sizes that you can mount with rubber cords or bolts with rubber sealing washers or silicone.
Possibly the best avail would be a case from Pelican. Water tight & virtually indestructable. Just make sure your sitting down when you check the prices.
<a href="http://pelican.com/cases_detail.php?Case=1780T" target="_blank">http://pelican.com/cases_detail.php?Case=1780T</a>

 

[H4]

In that case just google atv rear storage and check out local dealers to see whats avail from the different mfgs to find one with the features you want. You can then look for comments on a particular model as there are so many to choose from.
Consider what you need. Size, rear seat or not, heated hand grips, built in lights, mounting method (straps or bolts), gaskets on lids, color, foam inserts, locking lids, gas can compartments..........see what I mean. Or you can go with the small truck bed style storage boxes.