Get two sets of "body shaping" nylons, get a sheet of poster board and outline the hip/butt area you'd like to enhance, get a piece of 2" thick foam (from a craft store, Michael's or Joann's), and using a $5 electric knife (from a thrift store), cut out the pattern out of the foam.
Taper the foam from center-ish to the edge...put on one of the nylons, and as you align the pads, put on the second nylons over everything.
Adjust as needed.
Total cost will be about $30, most of which is the foam...look for coupons (often there are 40% off from both stores, and they accept each other's coupons).
There are YouTube videos showing this process.