I'm worrying about this now, too. I'm thinking of 4" all around. Another issue is the placement of the faucets. The tub in your photo has an area where the faucets can be placed, but I will probably have a rectangular tub. I think this means that I will have to put the faucets on the deck?

Being in a majority cooling degree day climate, the aluminum frame is not as big a penalty in your region. If that gives you the look and dimensions you need, go that route.

@kandrewspa I agree the roller enclosure is bulkier in person than I realized and I'm not loving the look. That photo is in my office but the primary reason we bought these shades was to have blackouts in the TV room. We currently do have curtains there (on a rod mounted to the wall above/beyond the frame), but I'd prefer they remain open as decor and have shades do the actual light-blocking. If anyone has suggestions for a more suitable blackout shade style I'm all ears! My personal taste leans more minimal/scandinavian so IKEA's a go-to, but as you can see it doesn't always work with the traditional bones of my '40s home.

I always thought MDF was cheap crap. I'd never purchase MDF furniture. Yet, I have learned through 2 kitchen renovations that MDF is a different story in cabinetry because of the application. A kitchen has a lot of flux in temperature and moisture levels and MDF expands and contracts more easily and is more stable in this situation than many solid hardwoods. It is truly a preferred material and better for cabinets. I'm with you that chunky frames look kind of cheap. But be aware that what you are looking for is more of a custom door and you will pay the price. If the frame is skinnier are there different considerations that need to be taken into account to make it structurally sound? I don't know but I do know that what appears to be an aesthetic choice may also impact construction and you will want a company that knows how to do custom work for your needs.