We had a problem last year with one about large grapefruit size and couldn't get hold of a harness so used a spoon, I have to say I prefer the harness but, she did lamb at night unassisted through the spoon with no following prolapse after birth (so there is hope).
it was good to have a bag of stuff to hand, I'm sure you have it covered but we had a bottle of boiled water cooled (the cool water shrinks the prolapse back sometimes). some iodine and some surgical gloves, also sterile wipes if you have some.
main problem is infection, so watch when she sits down if she prolapses poos then
gets say hay on top and it drys, its a bugger to get off but it really is better to do
before it go's back in.
personally i would not feed her too much tonight keep her clean like you have
and go and get a harness tomorrow morning. if she prolapses again wash her, serilise stick it back and use the spoon. then get the harness .
hope that is at all usefull. good luck