What have you used for a tool box that fits under your Tonneau cover? I bought a Rough Country Low Profile hard cover, and ideally would like to fit a box or, at least, storage under it.
www.roughcountry.com

Hard Low Profile Bed Cover | Ram 1500 (19-23)/1500 TRX (21-23)
Protect your bed from weather and would-be thieves with Rough Country’s sleek Low Profile Hard Tri-Fold Tonneau Covers. These durable, aircraft grade aluminum bed covers feature a flush, low profile design that gives your truck a clean, sleek look while protecting the contents of your bed.