There is an inspiration photo for you from Patricia Colwell in this thread. A trough sideways sink and faucet.

https://www.gardenweb.com/discussions/6479743/turn-sink-on-side-for-narrow-space#29356726

Patricia's recommendation would fit - but it won't provide any closed storage if you need some (you've specified that it's for a guest bathroom - but not if it's a powder room vs. a full functioning bathroom). I think that it was a 24" wide sink.

You could use IKEA Sektion kitchen cabinets to make a vanity base that would fit your specifications. You would need to use the wall cabinets - they are 15.5" with door/drawer fronts - so, you could overhang your countertop slab by 1" and still be at the right depth. You could put different pieces together in order to have the longer 36" width.

People have also modified other IKEA cabinets in order to have the right depth (for example, many people have used IKEA Sektion base cabinets to make a vanity - and cut the depth down to 21" or less). IKEA has some narrow vanities - they might not be 16.5" - but I assume that the depth could be modified the same way that the Sektion base cabinets and Pax wardrobe cabinets can be).

I've seen quite a few IKEA bathrooms here on Houzz that you'd never know were made with IKEA components.

It seems like when you search for a 16.5" or less depth vanity, they seem to be a narrow width also. However, I haven't done a full search.

You can find similar models in different sizes...

Do you mean full guest bathroom or PR 2 very different needs I did have that as a PR so the need for much in the way of storage was not an issue that was also along time ago and now many chouces for this type of sink.

A local custom cabinet maker can create what you need.