Q: When is a shroud not a shroud?
A: When it doesn't cover the entire radiator.
What you have is not a shroud, it is more of a "blade guard". It has ZERO effect on cooling, as you have found out.
An actual fan shroud covers the entire finned cooling area of the radiator, and is funnel-shaped (not flat!) to direct the air flow. Doesn't matter if the fan is electrical or mechanical, if the shroud doesn't meet those two criteria (at a minimum), it is basically worthless.
At a minimum, see if you can adapt a stock-style fan shroud to your radiator.
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Use a fan blade that is no more than an inch or two smaller than the shroud opening, with as many blades as possible.