I agree, not much looks and feels better than an O/U, sidelock, double triggers, straight stocked to the owners dimensions. Woodward made some stunning examples as did Boss, but usually with a single trigger.

Not wanting to spend upwards of $50K for one of these gems to take afield, I have on order a Rizzini, 28 bore, DT, straight stock with 3X wood. Now this is not a best gun by any stretch, and it is side plated to boot. However the cost of this semi-bespoke gun is about $5K and delivery time is about six months.

I am not an expert, but this rather inexpensive designed Rizzini might hold some ideas or design towards your goal of building your O/U. Good luck.