First off you won't be able to find any tape that is appropriate at cvs, other than paper tape which just plain sucks.
You need a zonas style breathable cloth tape. You can get this at your vet, or buy in bulk online. You can use 1"-1.5".
When you say you are using the improved method are you referring to the one mentioned in this thread?
Also how old are your girls? Like jan said the 5/8 may be to much right now. You can switch to the 1/2 if they are still young and their ears are still tiny. Eventually you will want to switch back to 5/8 as the ears grow.
A properly inserted/taped post will not pop out. Make sure it is pushed in to that pocket. That's where it is bottomed out. Then while holding the post pull up on the ear. Not to tight, just enough to get tension. Then stick the ear to the backer rod. I would always wrap the bottom piece of tape first before releasing tension on the ear. Make sure you are not wrapping the tape too tight. Bottom piece is a little over finger tight. Top piece is finger tight.
If you are using tape instead of glue use only two pieces to secure the post to the ear. I used three at one point to get the ear standing a certain way, but for now two will do it.
After the posts are in you need to make the brace. Which is two pieces of tape connecting ear to ear. You stick the pieces together so they hold firm.
I hope this helps. I never used the glue method, but I know many who have and it works wonderfully. Just look at Alberts ears.