Both ETI and Beater are designed to fit OEM air box. Even though ETI'S web site says,"has been reported that it works with these aftermarket airbox's: HRC and Turn-One", I wouldn't trust a tight seal with Thorston or Turn-One open air boxes with these two tanks. That is why I had Matt at DHC build me a Mori-type close air-box for my Beater tank.
I chose an aluminum tank over a composite tank because of all the problems with my full Kevlar lined cabon fiber tank I had on my RC-51 for years. Fuel finally started breaking down the integrity of the tank, allowing it to flex.
Also, on hot summer days at Buttonwillow, my composite tank would get way hot to the touch. no matter how much venting.
Also with aluminum, I believe it crashes better than any composite and is easier to repair.
ETI says they can repair their damaged tanks, and the weight of their tanks is impressive.
ETI tank is $1,260.00 (external vent + $48.00)
Beater Tank can be bought from Webike.com Japan for about $2,000.00 but is not on the web site for RC-51 so it's custom made to order. Mine took about 4 months, maybe a little less, and shipping and import fees were $200.00 to the US.