Enjoying a little snack...

I've been feeling a little bad lately that we don't have an age-appropriate high chair for Fiona. She really had no interest in eating "baby food," but now that she can pick things up and feed herself she can't get enough. This is my temporary solution:

Unfortunately, my anal ways prevent me from allowing anything but dry foods on the window-sill...which may soon cause a fruit/vegetable deficiency in her diet. Sweet potato puffs count, right?

Anyone have a brilliant idea to keep a baby from sliding out of this?