Give yourself some credit you can build that sump!!!!!!!
I wouldn't look to Lowes or HD for my needs for acrylic.....I would look in the phone book for a plastics dealer and go that route....you can have all your parts cut to size and even have them finish the edges if you want to pay extra, but I wouldn't use anything under 1/4" for the sump and most acrylic builders might say 3/8", but I think that's overkill, but I honestly would eurobrace the top of the sump.....Check out melevsreef dot com and alot of good information there and alot of how to's as well.....