The requirement of producing WSU was a nice idea in AACW-I. It added some flavor and strategy to the game. In CW2 the way it works doesn't fit into the scheme of things and it's causing more issues than it's worth.
In the end it would not restrict either side from putting artillery into the field in Missouri or Arkansas. Even if you couldn't build them there you could easily build them elsewhere and send them in. No big difference.
BTW to build a depot you need 4 supply elements. How many units you need to put those 4 together doesn't matter. They all must be =>50% in strength (hits, not cohesion) and have some supply. Since the supply phase is before the build phase, even if your supply elements are empty at the end of the last turn (when you order a depot to be built) they will probably have some supply by the time they start to build;
probably 
.
Note: land supply units are very expensive compared to naval supply units compared to riverine supply units. If you can use riverine supply units to build your depot, use them. If you can use naval supply units, use those. If not you will have to bite-the-bullet and use the expensive land supply units. BTW2 harbors with a depot are excellent supply sources. There's a reason the war followed the major rivers nearly everywhere.
To build a fort, again you need 4 supply elements, but this time plus 2 batteries of artillery. The battery can be any type of artillery, even captured batteries; so use the cheapest ones you can get your hands on; generally 6lb-ers. Once you start building the fort, the units that you have committed to the build are returned to your build pool.
You do not get the artillery in the fort; if you want artillery in your fort you must build and transport these in addition to the ones you use for building. The same goes for a garrison. Tip:
Don't use captured artillery or supply units to build with. Once you have used them they are returned to the building faction's build pool.