Just get a 1 gallon water jug, hold it at tank top level, turn on canister and click stopwatch, turn off and click stopwatch when it is full.
An XP3 is about $130 off eBay.
There's such a thing as hardcore DIYer; that you are not. A hardcore DIYer investigates what other people have done and improves on it. A NOOB DIYer tries to reinvent the wheel, says he's going for inexpensive, then makes it more expensive than what you can buy store bought item of better quality.
And don't even bother questioning my level of DIY. I've been building filters, tanks, and other aquarium related DIY projects for over 25 years.