I have a large fullsize one piece tub/shower surround from home depot. No cracks yet in 3 years of use but I made sure to brace it very well at side walls, I also shimmed the floor of the tub with small strips of wood to ensure even distribution of weight.
I actually made a mistake in measuring the unit when I purchased it for my bus and ended up having to cut a hole in the roof to drop it in!!
In search of Bus #2
(Sold)1989 Chevy Thomas 8.2 Detroit Allison AT545 Custom 3rd door added in back.
GM Service Tech