Since I've got full electrical in mine, people ask about my avionics box. As you can see from the picture, it robs a considerable amount of leg space. There's clearance on either side so your legs are clear of the box while sitting, but there's really no way to move your legs around on long flights. This leads to some cramping. The battery is mounted below the box, and when electrical work is needed, I disconnect it via the belly panel below.
If I were do build one myself (this one came with the plane), I'd try make it narrower for more space.