There's lots of ways to make those slots. Easiest is with a dremel. Drill two holes the width of your Velcro straps. Then use a cutoff wheel to connect them. Dremel also has a router bit you can use to connect the 2 holes but cutting a straight line with it isn't always easy. It definitely takes practice and a steady hand so the router bit may not be the best choice for a dremel newbie.

If you don't have a dremel, mark a straight line on the chassis where your straps will go. Then drill a series of holes as close to each other as possible. You can force a normal drill bit to drill sideways into the hole next to it if you do it right. With the bit in the hole apply some pressure on the bit in the direction of the hole next to it and move the bit in and out of the hole while drilling. It probably won't be perfectly straight so only do this to enough holes so you can get a hacksaw blade in there. A hacksaw blade will cut cleaner and straighter, wrap one end in some tape for a handle. Then use the blade to connect the rest of the holes and make the slot wider. Go slow and don't use much pressure, let the blade do the work.