I’ve always liked Charles Rogers daybeds. They’re available in a 39” twin size and a narrower 33” size which is more comfortable to sit on.

Just re-read your post and noticed you’re looking for a full size daybed, so my previous post won’t be helpful.

I'm going to disagree with those who say to get a sofa bed. A day bed with a good mattress will be much more comfortable for guests to sleep on. I don't know of any full size day beds, though.

Are there cabinet makers in your area? Years ago I had a local wood worker make me exactly what I wanted; it has a slightly recessed top to hold the mattress, and underneath piano hinges that allow for a lift-lid with full interior storage. Mine is not full sized.

I've had a lot of customers happy with this day bed from Lee Industries. It has a trundle which makes it a king size or two twins. I know you are looking for a full, but I think that will take up a lot of extra space unnecessarily when it's not in use.