First of all, I love mods and I love the OP's and subsequent posters' creativity.

That said, I do wonder, before I consider modding something, if it is really necessary. In this case I personally wouldn't bother. Why do you think straps would make it so much better as to warrant the trouble? You know on which side your clothes are. It is the bottom side anyway. So you just lay down the suitcase on that side and open the rest to the top as seen in the pictures. This way nothing will fall out.

Besides, straps aren't really that great at retaining luggage anyway. The more elegant and in your case more practical solution would be a "sail". Basically you take a rectangular piece of cloth which has the measures of that compartment. You sew velcro to the short sides of it and attach the sail/cover/panel to the velcro already in place. That will hold the clothes in place perfectly. If you use an elastic material for the sail you even get a slight compression effect. Since the velcro will have a much larger contact area, it will not rip off as smaller straps would under tension. It is also not much more work if at all because the number of sewing steps is the same. It will look cleaner and no real intrusions into the bag need to be made. It might even look "stock" if you find a piece of cloth in a matching color. Plus it is fully reversible and the added weight will be minimal.

That's how I'd do it if I'd do it at all.

Till