Would this give you a wide enough opening to clamp to the top or middle shelf? https://www.amazon.com/APPHOME-Foldable-Headphone-Stand-Hook/dp/B07PFV15WM
Why complicate things? Maybe just lay cover on the floor....behind the speaker (or along inside edge)? It's what I do. Simple no cost solution. Keeps it out of the way, zero risk of damage.Your carpet will protect it. Photo shows no foot traffic there. You can lay cover on side to minimize floor space usage....if it's an issue.If you must mount it, maybe a couple of flat bar braces, 12 inches or so, attached to shelf underside with double sided mounting tape, and protruding out. Can hang cover on those. To protect cover, can soften the braces by gluing on felt, or thin foam, or similar. Cheap solution. No damage to rack.This is similar to previous post that suggested dowels drilled and glued into wood block, and mounted to shelf underside. I like this approach, but it's a bit more work. Cost is likely similar.https://www.amazon.com/Alise-Stainless-Straight-Brackets-J7350B-4P/dp/B0C5RHXM69?th=1https://www.amazon.com/Adhesive-Indoor-outdoor-securing-carpets/dp/B0DP2B23NP/ref=wlr_d_sccl_1_5/144-3720177-8610928?pd_rd_w=g2EZq&content-id=amzn1.sym.93076504-0115-4aa1-909d-9ab8cc14586b&pf_rd_p=93076504-0115-4aa1-909d-9ab8cc14586b&pf_rd_r=S9BGBNVZZYHVJ1S4S806&pd_rd_wg=Its0X&pd_rd_r=667cb19c-deb4-4359-ac64-5dff4ecdd9b5&pd_rd_i=B0DP2B23NP&th=1
You can always put the cover in a Trader Joe's canvas bag and hang it on a door knob or in a closet.