A lot of racers just carry car shades. Extremely light, cheap durable and can be stuffed anywhere. Slightly more comfy than nothing and shockingly warm to sleep on. Also useful for sitting on when the ground is too hot/wet/cold/having a picnic... And they can protect your inflatable mattress too from sharp objects
Another option is a self inflatable air mattress. Couple puffs of air and your good to go.