I put a dropped shower in my bus, too. Here's where I started on that.
It's a very exclusive club!
Without welding, I'm honestly not sure how you would even do something like this. You could build the box entirely out of pieces of sheet metal bolted together, but without a welder you would at least need a sheet metal brake, and one strong enough to do 16 ga. would probably cost more than a welder.
I think $500 might be a reasonable price to do something like this (that's probably about what I would charge if you could make it to Philly), but that wouldn't include anything other than cutting the hole in the floor, fabricating the box and then welding it in place. If these shops are quoting $1500 to $2000 for all that plus painting and sealing and installing an actual shower with plumbing etc. then those aren't bad prices at all.
Rather than spending $2000+ to have a shop do this, you would be well-served to buy a welding machine for a couple of hundred and just do this yourself. You absolutely can do something like this.