Are you sure restricting flow will increase the PSI the desired amount?? This will work on a POSITIVE displacement pump. However most of these submersible pond pumps are CENTRIFUGAL and are limited in PSI but not GPM.
Remember the GPM rating on Pumps is OPEN FLOW with NO back pressure. Add just a touch of back pressure and Flow can drop by half very quick. My point being Do you have the Right pump??
At the price of a Screw on PVC Cap I would buy several. Of course I would Drill each different sizes. By installing a slip to IP thread you can quickly change caps until you find the correct size.
BTW I have made my own fountain nozzles with multiply sprays with just a PVC cap and a drill. Yes it is a trail and error process.
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