the rack that i like the best is made by soma- but not for mtbs. it's wide and deep
and you can get a fence for it- but you can't bolt it to the brake braze-ons (telescopic fork).
it does have horizontal rails underneath for small panniers!
i've looked at old man mountain (too narrow) and their mounting system is great.
i would like to carry a 20lb bag of charcoal or 10 frozen pizzas, a 10lb bag of potatoes,
a milk crate, etc.