There are several copper fin and steel fin manufacturers that sell base board with and without covers. Slant fin is one of many, many companies.
Depending on how much efficiency and quality which type your looking to buy.
Some have as little as 15 fins to the inch others have 30 and then the size of the fins also contributes to heat transfer.
Now to your original question about the return piping.
Having a decent quality base board you can either put the return below the supply fin tube OR what I did on several jobs with good results is put the return line just above the fin as I didn’t want very cool return water entering the cast iron boiler to possibly thermo shock it (On long runs the return DELTA* could be off by a lot of degrees)
Delta* The difference of return and supply water should be around 15 degrees BUT this is not always possible. so tempering the water may be a GREAT IDEA to bring it higher up
Contact any plumbing supply NOT A HOME CENTER and ask for fin tube publications. As the piping/ tubing can be from as small as 5/8 copper (OD) to 2″ + dia (steel used for hydronics and steam applications)