for 30-50 euros one can buy an inflatable mattress with electric pump (double bed size for 70euros).

I know my boxed airbed kit weighs 3-5kg, but I have travelled/flown with and used to avoid an excessive rollaway bed fee, and/or where hotel will not offer, extra bed, just so some visting good friends can share my hotel room on the 1x Saturday night during a week stay and not struggle with non-existant early morning trains/metro's etc when taxi fare is too much due to distance involved.