There are many possiblilties for this but the best solution I found is one offered by Moxiware which provides a good ipad waterproof case that you can mount pretty much anywhere you want. I mounted mine on a rail. The mount is compatible with ram mounts so if you already have these you can just buy the case.
Here is the link to the case I bought.
Moxiware
Here is the link to the case I bought.
Moxiware