Lay out all the things you will need for your portable office setup. It's likely you will need a laptop, mobile phone and power adapters. You may also need a portable printer, portable mouse, tablet computer, pens and paper, tools, samples, business cards and brochures, and various computer connections (e.g., network cables). Don't take anything that you can do without. If you use it one time out of a hundred you have carried it around for no reason 99 times. Keep your kit light and lean.
Measure the inside dimensions of your roller bag, note the internal height, width and depth. Cut two Velcro sheets to match the height and width dimensions. Hot glue them to the inside of the case, one on each side.
Test fit your items into the case with it lying open. Fit the laptop in the case inside a protective sleeve. Once you have things laid out in a way that it will all fit, cut lengths of Velcro straps for each item. Wrap cords and tools with the hook (scratchy) side in and the loop side out. Then press the wrapped bundles to the Velcro sheeting inside the case.
Cut two extra-long straps of Velcro that will each wrap all the way around the laptop case. Wrap them around the case about a third of the way from each side. Then press the laptop case to the Velcro sheet to hold it in place. Use this same technique for other large items such as a printer that you might need. You can undo just the strap of Velcro to release the laptop or any of the other items.