I know PVC pipe isn't cheap but there's usually off cuts in bins around construction sites.
I made these because I needed tall pots to accommodate for long taproots such as the ones on date palms, and take up a lot less space than big fat pots. the cool thing about this idea is you can make any length you require.
Step 1: Get Parts
Saw, hair dryer/heat gun, pen, tape measure, some timber off cuts, knife
Step 2: Mark Out
Measure diameter, divide by 2 and minus a few mill and make a mark that distance down the pipe.
Use strip of card with straight edge to make a line around at your mark.
Make 8 equal segment marks around the top of the pipe then another 8 around your line but opposite/In between,
Join the dots..
Step 3: Cut
Set up in a vice using a peice of wood inside to secure it,
Cut triangles out
Step 4: Make a Heat Barrier Strip
Cut a slot into a scrap peice of wood to line up on bend lines and heat with hair dryer on hot
Step 5: Bend Triangles In
Heat along the line (15 seconds ish) and bend the triangles in hold them til they set in-place I did 2 at a time
Step 6: Plans Seedlings
These are perfect for date palms as the have a long taproot and you can pack a bunch into a milk crate