I actually don't bother with a sand bath for Syrians if they've got a sand toilet. They don't seem so bothered about sand baths really. Robos definitely need a sand bath. Ours do the same - use the toilet as a toilet and a sand bath.
With a shoebox house - keep the lid as the roof and cut the base out, so it's open underneath. Then you can take the roof off to check inside so the nest doesn't get disturbed or fall apart. You need to take the roof off anyway to lift the toilet out - makes it very handy! The best place to cut a hole for a door is on one of the long sides near one end. So the other end is darker. They always nest at the darker end and pee in the corner opposite the doorway - so the place to put the litter tray is at the back corner opposite the doorway
Putting a bendy bridge over the doorway means he's less likely to block off the doorway and mess up the toilet!
If you don't like the look of the shoebox you could disguise it maybe - eg - put a piece of hemp mat on the roof or stick some round the sides or something.