Tartine-style in Convection Oven

I am starting a micro-bakery, baking 400g tartine-esque boules in a convection oven, so using a Dutch oven is out of the question. Baking stones are also not feasible right now for various reasons but perhaps you can convince me. Tenting with foil and adding water to the bottom of the oven is working pretty well for steam but I'm still finding that to get them JUST cooked in the middle, the outside (including the bottom)is getting over done. I have lowered the temp to 450 (oven is accurate) and still have this problem. Solutions?