Note I've collected all my changes here:
https://www.thingiverse.com/thing:5794841
Please follow that link to see my most up-to-date collection.

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Pictures below

As ever, big thanks to [ten16] for original post.

In this remix I've attempted to improve the break / lock feature on the rigid heddle loom. After a few of the original pegs snapped from sideways forces when locking the handle, I've come up with this: an arm fastened to a neighboring block, this new block doubles as a tension-rod holder and is positioned with help from a spacer ring between it and the 'handle' block.

(a) Break Arm 1 for the front handle (print 1)
(b) Break Arm 2 for the rear handle (print 1)
(b) Break Block / tension rod cap (print 2)
(c) Spacer ring -- a solid ring (print 2)
(d) Fastener Ring -- a ring with drill-holes prepared (print 2)
(e) Requires a 3" long 7/8" thick dowel (make 2, one for each block)

I've printed it. It works. Love it. I'd consider extending the tip of the break arm so one can pop it up with a thumb, but works fine for me as is.

So far it has been reliable. Will update if needed.

Look closely and you'll see a crack in the arm where it fastens. This is because I connected this block tilted slightly inward. The breaker block (green) is not properly aligned (vertically) with the handle block (blue.) It's very close, but apparently it makes a big difference. The arm still works fine.

This inspires me to stop using round dowels for the long bars and instead use square dowels--Home Depot has 1/2 in. x 36 in. Pine Square Dowel for about $2, for example.

This, of course, means turning those round holes into square holes and reprinting everything. But It may be worth it for future builds to have an easier time with assembly.