You mentioned you wanted to avoid drilling, so, I thought I would mention our simple and effective shelves. As you probably know, Scamp and Boler
closets are nearly identical. We recently to added shelves into our empty closet.
We are also replacing the floor so had some extra fibreglass (Epoxy Resin actually, but either works) on hand. The simplest solution for us was to cut some wooden blocks (plywood, cut about 1 inch by 6 inches) and then place them along the sides (and you could do the back too, if you are worried about stability) of our closet horizontally in three rows. I adhered them with thickened Epoxy. Then, let them dry and covered them with a thinner layer of epoxy, I will later sand and paint
so they look like part of the walls. (We plan to paint
the closet interior anyways, the blocks look nice as is, a bit plain Jane but functional.)
I then cut some plywood shelves to sit on the blocks. It took a bit of maneuvering to get the plywood onto the blocks but they sit securely and can be removed if we need more space. The little blocks themselves are on there and I don't think they are coming off without wrecking the closet now, so if you don't want something permanent - Epoxy is not the way to go!
We will later put in some baskets that we can pull in and out.
Sorry I haven't taken pictures yet, if my explanation doesn't make sense and you are interested, let me know and I will.