I like the innertube tire idea, will have to remember that one.
I accidentally taught my rope horse to stand when the reins are hooked over the saddle horn (I use 8' reins, so they're not tight while doing so). It's quite nice to not have go tie him up somewhere, or unhook reins, when I'm just jumping off to pick my rope up after a run. Haven't gotten my nerve up to see how long he'll stand, though am able to put the arena drag up without him taking off.