4" NPSH Female x 2.5" NPSH Male Adapter Aluminum FM30F25T Industrial & Fire 6” NPT

Industrial & Fire 6” NPT Female x 3" NPT Male Fitting Anodized Aluminum

So we couldn't be cheap about this part

This male 2-1/2" camlock fitting will fit into a 2-1/2" female camlock fitting

Fire & Industrial 6” NPSH x 6” NPT Double Male Fitting Anodized Aluminum

Downsides are they have a very low water flow - around 20 gallons per hour

4" NPSH Female x 2.5" NPSH Male Adapter Aluminum FM30F25T Industrial & Fire 6” NPTSpecifications Female Side 1 4 NPSH Female (National Pipe Straight Hose) Male Side 2 2. 5 NPSH Female (National Pipe Straight Hose) Working Pressure 300PSI Max Test Pressure 1000PSI Max Material T 6061 Aluminum (Aircraft Grade) Finishing Anodized (99. 9% Lead Free) NFPA Standards 1963, 4. 12, 5. 1 5. 6. 4 (Corrosion Resistance) Military Standards MIL A 8625 Type III, Class 1 Industrial Standards International ASTM B221 UL Labs Standards 236 8. 1