I'm still waiting to test my current incarnation of a sled but I had good luck last year with:
two narrow pvc tubes, crisscrossed and connected to the sled. I ran rope through the PVC and drilled a hole in the front of the sled and tied them on. I used biners to clip the other end to my packbelt. Worked well, but:
The cord showed signs of fraying where tied ot the sled and on down hills the X frame would slide forward of me and once to the side.
This past spring I changed the attachment points to the sled to stainless eye bolts backed by washers- one set in the front, one set on the sides. I intend to reinforce the sled (I'm worried about the plastic getting brittle) with thin Aluminum sheeting epoxied onto the sled.
I'm keeping the X configuration, if I have problems with too much flex, I'm going to add rope, or wire bracing half way up and down from the cross point of the PCV X.
The sled was bought at modells, slots for webbing were cut on the perimeter to facilitate the lashing down of gear. I'm also going to put everything in a duffle-bag, the sled tends to collect snow.