Ok, my crumble topping recipe is 9 oz. self-raising flour, 6 oz. butter, and 6 oz. demerara sugar (or caster if that's all you have). I usually put the flour and butter in the food processor until it looks like fine crumbs, then add the sugar and pulse to mix. This makes a LOT, but I just toss what I don't use in the freezer. I don't bother thawing it--just plop it on top of the rhubarb.
For some reason I'm not that fond of oats in rhubarb crumble, although I love them with other fruits.

eta: I find that the deeper the layer of fruit is, the soupier it gets. Maybe try a shallower dish?