I made one to piggy back on the top of the rolling machine suitcase. I just laid my biggest thing on top of the fabric and approximated how big it needs to be. I would make it a bit bigger next time since my portable cutting/ironing mat and the embroidery section of the machine don't both fit at the same time. It is just like making a pillow case with a squared off bottom and a flip over top. I will try to get a picture later. I did put a couple of bands across the back to slip over the handle of the roller bag.

BtW, some folks at my sit and sew use rolling egg crates from Homer D Poe or Lowes for their stuff and they seem to work well. I expect they are pretty inexpensive.