I think that the sheet would go over the load and the wagon sides, rather than be "tucked in" inside the wagon sides? Some info here: http://myweb.tiscali...ads/9-tarps.htm but a quick Google didn't find examples images of real wagons.
Perhaps your "modeller's licence" to allow the load to be removable?
One does see both arrangements in photos, although over the sides is much more common. It depends on the load - if it needs protection from rain etc., then over the sides - and ideally not with a dip in the middle, though there are plenty of examples! Some loads were double-sheeted, once with the sheet tucked inside the wagon, then a second sheet over the outside.
This can be viewed two ways - either frustration at covering up one's handiwork, or, with more planning, an opportunity to be lazy...