Grab some muslin, and do a drape.
Or measure, I suppose. If you must.
Seriously, though: you need a shape that mimics the bottom, and a strip that is the height of the basket plus some for seam allowance and some for upper hem. Attach pockets and pouches to the strip, sew it into a round, hem the upper edge, and sew it to the bottom shape, then slap it in the basket, and tack it in with a few stitches around the upper edge.