Greetings Max,
I found the Canon Tele extender a good solid alternative to a barlow for the reasons you originally stated. The price is 1 F stop for daytime stuff but I assume this also applies for prime photography.
I have a 1.4x converter and use it as a good tool for getting just a bit extra so if you go down that line I would love to see your results.
Just remember to cover the terminals on the tele extender on the camera side to fool the EOS into thinking there is nothing there otherwise you'll get the error.
Stacking is the way to go and then if there is a little too much background light, a fellow member Atalas
http://www.iceinspace.com.au/forum/s...ad.php?t=68095 has some very good photoshop tutorials one of which shows how to reduce this effect by shifting your dark offset in PS.
Good luck and would love to see how you go.
Cheers,
Darrin...