This is the one I use and I have had no problems with it.
http://www.clotilde.com/detail.html?...le&code=390736
I hope this helps, I really don't like the effect I got from sewing them together.

Here is what I have used to piece batting together "Heat Press" by Jeanne Harwood Designs heatpressbattingtogether.com - I bought it at my LQS

I buy iron on lightweight tricot interfacing, cut into 1" strips and then butt the batting together and iron down the interfacing. Does not leave a ridge, very smooth.

I have been zig zagging batting together for years. Never understood the need for the tape, as a wide zig zag works just as well.

Thanks ladies! I'll look into the tape and also try the zig-zag stitching.

I use Heat Press. I use it alot for lap quilts. Never had a problem

I like the heat press too, works great.

I've had really good luck with the wide zig zag, but I also use my walking foot, it seems to help the process a lot!

i would not iron poly batting, i lay one edge on top of another and then cut together so the edges matches perfectly...then i whipstitch really big (2") sts... it holds it together just fine without flattening the batting. i can do this in 5 minutes, all across a full width... when working with long pcs, just stake them out on the ironing board while you whipstitch together...

just butt the edges together and zig zag it works for me