Wallpaper paste it to some light weight wood...I don't know...I'd have to look, but I'm thinking luan like the skin they use on hollow core doors. Or mod podge it on. It's not like it's something precious that needs gentle handling. Cut the wood to either fit the poster or leave a narrow margin of wood showing around it.

See, eg this from Home Depot.

I used these for a Longwood marketing poster that was about the same size.

https://www.wearewellmade.com/products/premium-magnetic-framing?variant=45476893720812

eta: i didnt realize 70” was the width nvm my suggestion unless it could hang vertically.

I take posters to a framer and ask them to vacuum mount them to foam board. The I get an adhesive hanger and attach it to the back.