If you want to add covers, just roll with TLA's sandbag style covers.
Which means you need empty sacks and a shovel to construct one. This can be done anywhere, but keep in mind that every sandbag has a rather heavy weight (20-60kg). This makes carrying sacks and a spade more attractive than carrying a bunch of filled bags.
Multiple sandbags can be added to one hex. One or two sandbags block movement but allow sight and gunfire, three or more sandbags block both movement, gunfire and sight(?).
The amount of sandbags is limited to 100 bags, each bag has 100hp. You can remove bags by using science or a shovel on them. Adding or removing bags works one by one, so an enemy trying to remove your fortification will need some time to do so.
Last but not least: Sacks can be made out of a brahmin hide and a rope or some fiber.