Ziplines are really fun, until you hit the end and stop short. Brakes can solve this but they are around $80 (http://stores.backyardziplines.com/-strse-17/Brake-Block-Kit/Detail.bok) Why pay that when you can make your own for about $20

Step 1: Materials/tools

MATERIALS
zip ties x2
6x6x1 block of wood x2
2 1/2-3in bolts (it doesn't matter) x2
20-30 ft. of 1/2in. bungee cord (depending on the and speed of the zipline: http://stores.backyardziplines.com/-strse-1/1-fdsh-2%22-Bungee-Rope/Detail.bok)

TOOLS
large screwdriver (only if you are using an eye loop)
small bar clamp
wrench (for the bolts)
eye loop (or something to anchor the brake to)
a hammer
a powerdrill with a large bit
a tablesaw with a dado (or something to cut a slit in the wood with)
a ladder
a person to test the brake

Step 2: The Block

use the dado to cut a slit in the wood, make it slightly bigger than the cable. do this in the center of both blocks.

Step 3: The Knot

use the drill to make a hole in the wood so the knot can fit, if you need to go through the wood just start a hole in the other block. you should be able to put the knot in the hole and put it together without a gap. after that use the dado to cut a 1/2 in. slit for the bungee in one of the blocks.
Optional: you can use a zip tie around the knot if you are worried about it coming un-done

Step 4: The Bolt

line up the blocks of wood and drill 2 holes big enough for the bolts. i put one in each corner, but you can put then in all 4 corners.

Step 5: Put It Together

now go wherever it is easy to work on the zipline. put the knot in the knot hole and and do the same for the cable. put the top peice on and use the clamp to hold it so you can bolt it together.

Step 6: Find a Spot

find a spot for the brake block. if you used a 20' cable then you want it 10'-15' away. if you used anything bigger than remember that the cable stretches about 150% of its length. now find a place to anchor it(it could be a fencepost or a stake in the ground or anything that wont move under a lot of pressure) the bungee shouldn't stretch out when it is anchored.

Step 7: Anchor It

now anchor it im putting an eye loop in a fencepost, but you can do whatever works for you.

Step 8: Test It

this is probably the best step. just ride the zipline and hit the brake to see if it works. if it doesn't then you may need a better anchor or the brahe to be further away from the end.