Wouldn’t any old waterproof bag do? Having had a Thule Easybag it wasn’t much more than that. OK it had a bit of plastic reinforcement on the edges of the base and straps to connect to the Thule Easybase (so useless for your bike rack) and a nifty but useless lockable zip (useless in that if you really wanted to get in the bag a Stanley knife would have bypassed the lockable zip). Any waterproof bag with straps to strap to your bike rack would do the job almost as well.
But looking at the price of those North Face waterproof duffle bags and their like the Easybag is a good price £120 for 95 ltrs vs. £177 for 315 ltrs). But then comparing to North Face pricing is hardly demonstrating it’s a bargain! I think Decathlon does the water proof bags…maybe a couple or three of those might do?