My friend designed a fantastic solution for diverting rain water in tight spaces and this can also be converted to perforated tube section to make custom French Drain. 

 Rainwater reducer description

I wanted to connect a standard OD ø68 mm rainwater downpipe into a standard ID ø25 mm flexible aquatic hose (/Corrugated Pond Pump Pipe). My application to route rainwater collected from the roof underground via the cheap/easily buried flexible hose. I call it micro-bore drainage 😂

This printed reducer design relies on a flexible print material (e.g. TPU), to enable compression fits and intent is to use jubilee clips around each interface to secure the connections.

Flow rate testing: tests proved that a 15 m run of the ø25 mm tube laid horizontally, could shift 25 litres per minute! This assumed that the attached vertical rainwater downpipe was able to accumulate ~2 meters height of water (pressure) if necessary. 

Application: A study of our rainfall and roof area showed that 25 litres/minute flow rate was sufficient for “heavy rain”, but each application should be assessed separately. (Roof area can be easily measured with Google Maps, and historical rainfall can be accessed using Open Meteo).

If anyone wants to print this reducer, you should:

• Print using a flexible material (e.g. TPU) - this allows the jubilee clips to create compression seal
• Deburr edges of the fitting tubes to ensure easy fit
• Lubricate surfaces before sliding together
• Lock with a jubilee clip (to prevent folding, take care not to overtighten)
• Fit a filter at the top of the downpipe to ensure that detritus from the roof does not enter the ø25 mm tube which would block it. And the filter should be cleaned on some kind of maintenance schedule.