No rubber mulch, they're starting to tenuously link to that to causing cancer. Also think it would move around too much to be helpful for this purpose at all, and probably not heavy enough.
Shredded wood mulch would be the heaviest, best able to block light and add weight, least expensive. Overlap the seams of the cardboard well. Don't try to stretch it, wait until you can find/get more if necessary. If you call a mom'n'pop furniture or appliance store, they should be able to help you get some large cardboard. Just remove any tape or staples before using.
If there are other types of roundup besides glyphosate, it's not something I know about, have never read anything about that name of product having other active ingredients. Sorry, I don't have an answer for that, except that your story of it not working, whatever it is or says to do, would probably make me give up trying it.
Elaine, agreed - I didn't even get into the beneficial microbiology of using cardboard and organic mulch for the smother! It's lovely when killing weeds and building 'good dirt' are accomplished from the same action.
