I have had success with the "rugby tackle with a big towel" approach - but only with up to "half-grown" cats - my hubby keeps warning me that one day I'll get scratched / bitten etc - so far I've been lucky.

The suggestion with the pet carrier being covered sounds good - you just need lots of patience (which I'm sure you have), and a calm atmosphere (which it seems is SOMEWHAT lacking)

Such a pity you can't get your hands on a humane trap - they are very effective. (If you DO catch them in a trap or a carrier, have an old towel or something to cover it with - this calms them and there is less chance they will injure themselves.

Best of luck - we are holding thumbs (and paws) for you