How to choose a neutral 18% grey backdrop?

[Two_In]

I need a neutral 18% grey backdrop. There's a lot for sale. What should I choose?

I'd need something that's robust enough for fairly frequent use - more than several times a year. And for travel - would need to fit in back of car (seats down possible) with other equipment.

It would need to roll down onto the floor for a continuous background. Dimensions from top to floor (bit rolled onto floor would be extra) would be no less than 150cm x 150cm, but much larger not needed. To be used Smartflash and softbox - or outdoors, weather permitting.

Thanks for any help.