Very simple: don't cover it at all, ever.
There is a reason for the hole being there. It is there to let stuff out! Usually clean steam, but if you loaded the cooker wrong, a bit of mess can come out. Then you should be happy that 1) it is coming out and not interfering with the cooking process inside, and 2) drawing your attention to the fact that you have loaded it wrong, so you know better next time. Just clean the mess.
In a pressure cooker, it is doubly important to not cover it! It is there as a safety mechanism, to prevent an explosion. Yes, I mean a real explosion, which will damage not just the pressure cooker, but also a good part of your kitchen! While pressure cookers made for home use are very safe, this depends on them being used as intended, and especially on not obstructing their safety valve in any way.
If you want a cleaner kitchen, simply position the rice cooker in a place where its steam hits something wipable, like a tiled wall. You can also put some kind of easy-washable foil on the underside of your upper cabinets above the cooker, or attach some multi-layer paper (an old magazine will do) which you then exchange regularly. But you have to live with the fact that you will have steam (with slightly dirty aerosol) coming out of that hole, that's what it is for.