Rita Rump Pockets with Minky polyester, hidden layer of antipill and inner of fleece?

Just bought a batch of 100% polyester minky at spotlight, super soft for outer. An anti pill polyfleece blanket i had lying about for a hidden layer: these two together would then be outer layer. and as an inner i found some flannel receiving blankets (5 rectangles of 75 x100 cm) which were 60% off

I am planning to stuff them with microfibre towels with a layer of flanel sewn over.

anyway
my question is

if i use both minky sewn together with the anti pill fleece, will this be water resistant? will i need an extra cover?

as the minky outer snappiable or will i need to use nappy pins?

i got some braided and some knitted elastic, which would be the best to use?

thanks for your replies.

it is just a little project i would like to do to keep my mind busy of certain things