Do you think spacers could be put in where it is bolted down?
Seat bolts are usually about an inch long, so maybe.
But, but in the Hondas I've owned, one side of the seatbelt is anchored to the seat frame. So, for the seatbelt to do its job, the seat needs to stay attached to the car. Putting spacers could result in less of the threads on the seat bolts making contact with the threads in the car's frame, leading to them pulling out in an accident. It would probably also change the geometry of the stress on the bolts in an accident and they might snap. I'm not saying these are definite risks, but worth thinking about. I don't think I'd do it.
There's probably some sound-insulating material under the carpet. Maybe try pulling the carpet up and removing everything under it on the passenger side. And get rid of the floormat on that side. Those steps might gain you an inch of foot-to-seat gap.
Or go to an auto upholstery shop to ask if they could pack some extra foam inside the lower seat cover.