If it is a mud bottom a danforth would work well and they are very light and inexpensive. You have to let out some line so there's enough angle for the two points dig into the bottom. Once the anchors set you can shorten the line a bit. Your boat will blow back-and-forth in an arc a little bit.

If you want to something that may hold straight down that's very inexpensive and you don't mind using it, a concrete block could work well. I used to use one in the current below McAlpin Dam all the time. It's a heave ho to get it up, I think there about 20 pounds. Good thing is if you lose it doesn't matter.