The batteries might fit in the gap laying down which would be even more hidden. I didn't do that because over time I imagined it might leak from one of the caps and I wouldn't know. If valleys were cut into the foam and maybe the terminal ends inverted so the cable was lower, that might hide the cables further I'll have another look at that.
My old engine bay terminal ends were wired into a junction box near the old battery location I also have a Ctek plug wired in there so I can plug the car in to charge near the fuse box under the bonnet. The long 0 guage positive cable was fed behind the ABS pump, through the firewall where the OE loom goes, along the passenger door sill and rear bench seat into the boot area near the suspension turret. The negatives can be seen bolted to the chassis in front of the spare tyre. I didn't have to strip much away, just the plastic door sill strip where the rear washer pipe can be found. I fed it past the rear speaker whilst that was out so didn't have to pull internal plastics away.