What type material are the existing bows ?

Steel/aluminum/wall thickness  ?

You could use a ring roller to form new curved bows that match the end caps out of either flat bar or pipe. Not sure if metal conduit would be strong enough or not ?

Need to start with a piece longer than needed as you lose some at the ends of each ring you roll plus you would need to account for the ends and how you will attach tot he box. 

I made some flat rollers for my HF ring roller using pieces of pipe that fit inside one another for rolling flat bar rings.

The ring roller only came with dies for various tube sizes. You can buy many other dies from this place but I just made something myself more as a proof of concept.

https://www.swagoffroad.com/SWAG-Off-Road-Tubing-Roller-Dies-Compatible-With-HULK-or-Harbor-Freight-Tube-Roller-_p_51.html

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