A dry fit of the BT-20 tubes showed the ends weren't really square.
On the left is the two tubes before sanding.
The ends were trued up with 220 grit on a sanding block.
The yellow coupler was way too tight in the BT-20 tubes.
The outside was taken down with 220 on a block.
The instructions say to make a tri-fold shock cord mount. In a BT-20 tube there not much room for the mount and a parachute.
I'll upgrade to Kevlar, tied to the upper half of the coupler.
A hole was punched with the rotary punch and a slot filed to the coupler end. The slot allows the Kevlar line to sit flat under the body tube without raising the tube wall.
The Kevlar was tied on with a triple overhand knot.
To gauge the length of the Kevlar, the coupler was set beside the longer BT-20 body tube.
An overhand knot was tied in the upper end of the Kevlar below the top of the tube.