Suggestions or should I just run it across the top?
If it was mine, I would pull the cord back to the wagon box and install a 7 pin socket connector in the cutout on the front of the wagon (same connector as used on the back and the wagon is cut on both ends for right/left unload options.)  Basically wire it the same as semi tractor/trailer  Then connect a coil cord to the connector you installed on the front of the wagon.  Next time you damage the cord, unplug and replace with a new one.

https://smile.amazon.com/dp/B000FBWPXY?psc=1&ref=ppx_yo2ov_dt_b_product_details

Easily under $100 repair.  We have all of our wagons wired this way and each tow vehicle carries it's own cord.  The one in the pickups has 7 pin round on one end and 7 pin RV on the other.  I have a couple extra of the 7 round/round for between the wagons when pulling in tandem.  The cords don't take much room and don't drag on the ground.

(Capture (full).PNG)

(7pin_socket (full).PNG)

(plug_holder (full).PNG)

Capture (full).PNG (37KB - 448 downloads)

7pin_socket (full).PNG (9KB - 485 downloads)

plug_holder (full).PNG (17KB - 443 downloads)