Car Top Carrier

What is the least expensive, yet secure way, to put an OK T 13 on top of my Honda accord?

2 Pool noodles or foam blocks and 2 cam straps.
Place the pool noodles on your roof then put the kayak on the pool noodles. 
Run the cam straps over the kayak and through your car doors. 
Tighten.   
Bow and stern straps  help especially if you re going on the highway.  If you can't find pool noodles or foam, rolled up beach towels work pretty well too. 

Everything ndogg said AND put the yak upside down.

I was using a set of J cradles (and may still if I have 2 yaks to carry), but I found it was very hard to top my T13 solo with the J cradles.

now I just have pool noodles zip-tied to the cross bars and use 2 cam straps to secure the yak. the cheap noodles look a little silly, but I doubt I can find dense, black closed cell foam for $2.

I also have a cable lock that I use for a little piece of mind.

Bow and stern lines absolutely ---  60 lbs of yak moving at 6o-70 mph is a deadly missile.   Not an option unless you're going on city streets for a mile or less.  Don't ask me how I know this....

I have found a aired up Bicycle Innertube works very well too.. as a cartop cushion..The straps thru the open doors are Very Hard on the weather stripping and Rain will Soak the inside of your veh. Very fast in the NW, front and rear tie downs help alot to keep your yak from turning or sliding to the side..

Alright - So I managed to attach a 20 yo Thule system - cross bars - to the top of my car.  So I need some pads for the bars, noodles, (I don't have) and some ties downs (I have some ratcheting ones)... how do I keep from messing up my paint on the front and rear of my car with the bow and stern tie downs?

Take a length of rubber tubing, any kind, and thread it through the bow and stern lines respectively. Then slide them into position over wherever it is that the rope would otherwise rub against the car.  I've used a length of rubber hose off hand-held nozzle I used to use in the tub to wash off the dog...  Very cheap, works great.

Pipe insulation works for the bow and stern lines.  Also use it under the buckles on the ratchet straps to reduce any gouging.