Elasticated cover for IKEA Hemnes day bed?

[Lulumush]

I have a day bed in our playroom which my kids sit on to watch telly. It has both the mattresses on and a red bed cover which makes it look more sofa like. The cover gets rucked up and doesn't stay tucked in ever - which is soooooo annoying!
does anyone with one of these day beds have any suggestions for a pretty cover ideally with an elasticated edge to keep it on the damn bed?

We use a fitted sheet. You need a Euro single size though, so will probably have to order online. For some bizarre reason best known only to them, Ikea don't sell fitted sheets that properly fit the hemnes day bed. Or they didn't last time I checked. If you've got both mattresses on you might need an extra deep one- we just have the one mattress on ours and the other down the back of the sofa.

Thanks TuckMyWin
I have the sheets (which incidentally are fitted!) - I am looking for more of a cover for the bed, which hides the sheets for when its not being used as a bed, if you get my drift. I'm using a large John Lewis bed cover at the moment but it is permanently un-tucked and a mess when the kids sit on it - hence the need for something elasticated....
I cant find anything on line so I may be looking for something that doesn't exist......

Could you make, or ask someone else to make, a large bag type of thing to put the mattresses in? A bit like a big duvet cover .