Two pieces of 1/2" "riser flex pipe", found in the irrigation section of a large home-improvement store, provided the extra reach down to the water for the filter plumbing.
This pipe has an outside diamter of 5/8", very close to the size of the rigid pipes included with the canister filter. It has the additional advantage of being black in color.
Clear flexible vinyl hose functioned as a sleeve, creating the union between the riser flex pipe and the filter pipes. Note the size used, a thick-walled hose with 5/8" inside diameter and 7/8" outside diameter.
This vinyl hose retains a strong curl from its storage on a roll. The pieces are easier to work with after straightening, which can be easily achieved with a dip in boiling hot water. Before this treatment I had cut the two sleeve pieces to slightly longer than the desired length of 4".
It is difficult to cut the hose end very straight. In order to get a cleaner cut I wrapped the hose with a piece of masking tape, then used the tape's outside edge as a guide for careful re-cutting.
In order to join the irrigation pipe to the J-shaped return fitting, I found it necessary to include this smaller piece.
This short section snapped onto the return fitting, and with removal of its small elbow piece it provided the right diameter for joining with the vinyl hose. The plastic elbow was only loosely bonded to the piece of pipe, so it came apart readily with a quick twist.
The next picture shows both the intake (light blue) and return (dark blue) fittings joined to the riser pipe with vinyl hose sleeves. Note the location of that short snap-fit pipe (arrow).
I found that the sleeves slid onto the pipe sections rather easily, although re-softening with very hot water can facilitate repositioning if the fit is very tight.