After spraining my ankle I found out exactly how hard it is to carry things while on crutches. I found a number of water bottle holders and similar things on Thingiverse but there weren't many simple mounting brackets. So I put together a quick one.

This will clip on to your crutch and bolt in from either side. You then have two 5mm hole with which to mount whatever you want. I mounted a box on there to hold things but you could mount a water bottle carrier, a clip, a hook, or anything else.