10mm or 11mm main rope. 11mm pictured, but my other saddle has a 10mm climbing rope bridge. I would start with no less than 6' (preferably 8') to make sure you have enough length. (Knots eat up rope quickly and you can always trim the excess)
8mm accessory cord. I would start with no less than 4' of cord (preferably 5') for the prusik. This allows you to tie a double fisherman's knot with sufficient tail and gives you a big enough loop for 3 or 4 wraps with spare change leftover.
Create your prusik loop from the accessory cord and wrap it onto your main line. Then girth hitch the prusik onto one of your bridge loops. At this point, you can tie the emergency backup knot onto the same bridge loop with the main line. I use an overhand on a bight for this, but use the follow thru method to tie it. (Remember to leave sufficient tail on all of your knots for safety purposes)
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Then you can tie the girth hitched overhand on a bight on the opposite bridge loop with the main line. This will also need to be done using the follow thru method, shown here:
I've been racking my brain on trying to find a way to girth hitch both sides of sit drag bridge with my 8mm rope, and I think I figured it out, but what some second opinions before trying it. What I did was tie a monkey fist at each end, double it over pass both ends through the loop. On the the...
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Having a girth hitched knot on both bridge loops provides better adjustability and comfort. It allows you to move the bridge up or down on the loops to create more or less butt-cupping, and when loaded with your weight, the girth hitches stay put.