Just go to any home improvement store and buy a security cable about 6 feet long with loops on both ends, it will cost about $12. Then buy a cheap combination lock like the Master Lock 175D for about $17.
It's not the highest level of protection but if you're worried about someone taking your bike while sleeping, and it has happened in parks, then that will stop them.
The lock system I recommended above is lightweight and easy to store in a pannier or handlebar bag.
I use the same setup when I go touring or bike camping, and it did stop 2 guys from stealing my bike at around 1 am on a dark rainy night in a campground where the next nearest tent was about 100 yards from my tent, they thought the noise of the rain would disguise their approach to my bike, but I heard them and watched, the one guy walked over to my bike and then went back to his friend and whispered that it was locked, and they left the area. I had locked the bike to the park bench that was on my site.
There are tiny little thin cable locks, and plastic locks that look like a wide zip tie, those are worthless, a pocket knife or a pair of pliers with a cutting edge can snip those fast in about one second, and do so without too much noise, and if you are a sound sleeper you'll never hear the person. I've been trained to sleep really well, but any noise that is not supposed to be there will wake me up in a flash, and I heard those 2 guys trying to walk quietly in a downpour rain storm on the park road, so I woke up right away and listened, then watched without them knowing I was watching. Most people can't do what I can do while sleeping, but even though I can do that, I don't leave anything to second chance.