The metal cage tank might as well be built with one wall - the viewing wall - of appropriately thick acrylic panel and the other 4 of thin acrylic "skin" like 1/2" or 3/8" or even 1/4" thick acrylic panels. No need for PE welder. Can eurobrace all the seams to improve strength and waterproofness.
I think the thread's focus is as cheap as possible and as easy as possible but functional and lasting.
Zoodiver
posted some of his local “behind the scenes” shark tanks with rather large windows here yrs ago. Ive most def. seen “stock tank windows” larger than 1’x2’… You could easily do 2’x4’ windows with 1/4” poly carb IMO… most people are working with smaller tanks to begin with so i think its more so it just hasn’t been done much or seen often. Most of the stock tanks available are short to begin with restricting the window heights. Sure there going to bow pushing the limits with a 3’x4’ or 3’x6’ window but, i think its very plausible. 4’x4’ miiight b pushin it… 2’x8’ or 3’x6’ would work fine. Another thought is poly carb is expensive so that is another reason why people may only be using smaller windows. Honestly ur proposed window dimensions may work fine… poly carb is 250 times the impact resistance of glass where as acrylic is only 17… pretty crazy… ill leave u with this very crude example for now while i search for better pics… this dunk tank window has to be 3’x3’ at the least lol… there also mass produced so u know they “work”… with glass calculations always going by height alone for material thickness, u really can push the limits to the extreme with 1/4” poly carb. Id do it in a heart beat if a reasonably sized stock tank fit in my basement ?… more “food for thought” is stacking two 1/4” sheets together. I bet u can do a 4’x8’ window span that way. Only reason to stack them is to curve the thin material easily while also doubling ur strength against bowing ?? 
My biggest concern with that would be how to remove the "Easy Dunker" decal. 
