Ones ive used are just a band of springy steel with a square type allen key bolt to do them up.

Only cost a few quid, and work just fine.

The key thing is keeping them in contact with the block during fitment, and not to force anything, if it feels like its not going to go then stop trying!

I use the wooden end of a hammer to very lightly tap my pistons down into the bores, literally just lift it up an inch or so and let it lightly tap them.