Hi Colin, for what it's worth I'll tell you my experience, in the early 1990's my (then) girlfriend made an exact copy of a Zone VI washer 20x24 (except she expanded it by a further 4 inches). For this we used second-hand (used) perspex (this is what we call your 'plexi-glass') and it is 6mm in width. 6mm is close enough to 1/4 inch. Anyway, I've used this washer constantly since then & have never had any troubles concerning it's lack of strength. We used the specific glue for plexi, the one where you should wear a mask while applying it. Also, I haven't had any problems with buckling, although I always keep a few inches of water in it when not in use. Maybe this vote of confidence with using 6mm material for 13-14 years will ease your anxiety.
david.