bedding for a P&T portacot

We have just used out Phil & Ted's portacot for the first time - it worked a treat, but I wasn't too sure what bedding to use with it. I just put a bottom sheet down, then baby in sleeping bag, and a top sheet and blanket on top. This is good for now (although the bottom sheet did move around a bit while she was asleep), but as she gets wrigglier I think she will start pulling at the sheets and blankets and I can't figure out how to secure them coz you can't tuck them in!

Any suggestions?