A simple conduit pipe coupler designed around cheap 20mm x 2000mm conduit available at Wickes UK as the other aluminum based option was too expensive. Due to weak strength of the pipes, I put them every 50cm which allows about 7 spools to fit.

It was my first design made in Fusion 360 so it's not perfect but seems to do the job.

I printed this in PETG and it seems quite sturdy though it does take a long time, about 3 hours for each one of those and about 130g of filament.