Good morning, Bob --
I just did the exact same thing recently (the firewood pallets). In fact, I am converting every heavy thing on my property to pallets. I am building a storage building that has a second floor that is specially designed to the lift height of my tractor and I will have a floor pallet jack on that second floor to move pallets once they are there. You might say that I am "palletizing" my property.
Now, back to your firewood pallets. Here's what I did, after trial and error:
Materials:
1. 48x40 pallets, with the two upside "U"s in them for the forks (make certain that you get this kind). In Central Texas, about $6 each new.
2. Roll of 3' high by 100' long rectangular hole fencing from TSC. About $50.
3. 2x4 treated lumber for corners (two at each corner, nailed to each other in an "L."
4. 1x material for top rails.
5. Use air staples to fasten fence to pallet first.
6. Then, air nail corners to pallet.
7. Then, air staple fence to corners.
8. Then, air staple rails to corners on inside, with fence sandwiched between.
9. Then, air staple fence to rails.
Very, very strong and long lasting.
Once you set up production, doesn't take too long to make. Results are long-term, and provide firewood at your home when needed, and stored away from home when not.