SHOCK CORDS ARE DANGEROUS.
S H O C K C O R D S A R E D A N G E R ---------0-]
search to Seattle Fabrics, find poly strapping and steel cam buckles, polyester thread, heavy duty needles, a hardwood square for pushing needles thru.
we all over here at sea kayaking/down river rafting use straps and cams. search to NRS.
Wal sells cam straps in RV. Good for light loads in unstressed conditions eg the interstate at 70 bringing a very soft feel for Royalex canoes/showboats but the DIY from SF is better or buy a string from NRS.
NRS strap loops holding the cam are sewn in an X bordered with a square.
An 18' kayak sits on my van's roof. Cam straps hold the yak down during tornadic microbursts with the yak moving fractions sounding squee squee on foam blocks. Straps are looped around yak then snugged.
BTW, if stuffing a rack bag is the goal, u-bolt plywood onto the rack top then add a military duffel from Campmor.com: max capacity at lowest most durable costs.
You make the u bolts with aluminum strap, bolting thru ply to straps over rack members.
An excellent test of rack supports strength's ! Sticky tires thru a slalom.
63210 at
http://www.campmor.com/outdoor/gear/...exsport+duffel