Im not sure what website, but one of those who sells piggy fleece liners also sells an extra fleece piece to go with it that draps over a grid like you described, to make a hiding place in the corner. Im not sure how they have it attached, but Im sure you can think of a way. lol Sounds like an easy way to make a cute hidey for your piggy.
so to make my forest fleece you can just get some fleece and cut it into strips only cutting it half way to the end of the piece. If you have the cubes on part of the top of the cage I then weaved the cut strips through the top squares and tied then at the end of the cube and hung over into the cage. and to make ones in the middle part I just tied individual strips of fleece onto a part of the top metal cube and it hangs down. Looks at my pics to get an idea. Hope this helped. As far as using that as their only hidey-house I would get them a pigloo or a wooden house as well because they like to hide in and on those. My female mom pig loves to perch herself on top of the wooden house and the baby girl likes to chew the window below. So its good for their teeth as well. I also just draped a piece of fleece over the grid to make a hidey-place and I just used a clip to make it stay. my baby girl pig loves to lay in her cuddle cup beneath the fleece that's draped.
I took all the houses out of my cage, now my two boys just have a fleece forest and on another corner a piece of fleece just draped. It works just as well, takes up so much less space and I think they may like it better.
Like @Inle_Rabbit I have no actual hideys, I had to take them out because my boys were fighting over them. Now I have to small fleece pieces hung over the corners. It gives them more running room.