I agree with the 2nd post. I.bought a 50' stainless steel one on eBay a while back (3/8" x 50'). Chills 5 gals in about 20 mins or less. More surface area is preferred amd most manufacturers use the 3/8 diameter.
Another thought, after pricing the DIY method vs just buying a premade one, it ended up costing about $10 more to.buloy one... To me, $75 vs $85 was worth the expense for a few reasons:
1) it looks awesome, unlike some diy chillers that look warped... functionality is the same, but I want my stuff to look good... afterall, this thing shouldast a decade or more.
2) The ease of clicking a button and its at my door in a week... the anticipation is fun, and I don't have to go all over the store to find what I want.
3) Location... it costs me about $10 to drive into town.
If ten bucks breaks the bank, then go for it. DIY projects always bring a sense of pride too. Ecomomically, it made sense for me to just eBay the sucker...