Can you provide a link to the warlmart item - i'm not sure waht 'polyester fill' are - i presume they are similar to filter media bags. I've been debating between using customaquarium sump or using a tank with swisstropical sponges. I'm not sure 40 gallon is a large enough sump for a 450 tank but certainly a 55 or 70 should work. Of course the pia would be drilling a drain hole at the end. The drain hole is so that if that a drip system could be used - and the out flow would be from the sump as it 'overflows'.
Those are called filter media cups and I got them on eBay from a seller that goes by the name of: filtermediacup. Original name lol… They work really well and sure beat cleaning socks. I just trash my polyester fill after a couple of days and throw in new. At $13 for a huge bag of it at Walmart it’s hard to beat.
My sump is under my tank. I fill from my sump and turn one pump on to keep up with the water flow, when the tank up top is full and starts overflowing back down to the sump then I fill to my desired water height in the sump which is dictated by the noise of the overflow and how long it takes to get the siphon going. The first time you fill it you will no doubt need someone at both the tank and sump to if nothing else check for leaks and make sure everything is flowing as it should. Good luck.