I made that in paint...

So what that means is if you put a single speed free wheel on there, your chain line will be off. Which will result in

1. your chain bucking off
2. friction that will make your bike very unificent

That being said, options you have include

1. setting the index point of your derailer to a larger cog of your freewheel, forgetting about cables etc.
2. buy a 24 inch single speed wheel
3. go to a thrift store, pick up a junk little kids mountain bike, and learn how to
a. remove freewheels
b. reset spacers
c. dish wheels (and truing them)

That would cost you about 10-15 dollars, which is basically what people throw away on fast food.