first off if you are mounting the phal you should do it naturally with the roots up and the plant hanging down.
I have not had good luck mounting keikis, probably because I can not devote the time necessary to water them properly.
A keiki has always been bare root, and I feel that it can be mounted directly without the sphagnum I used in the photos. Phals like to be moist all the time. I have good results with keikis that can go a week or more without water. Bare root requires regular watering, which is more attention then my phal keikis have been getting. (By the way I have only one test keiki and it is living but not growing.)
The plants in the photo have sphag in order to give the plant a trandition period from a pot to a mount. I do not see why you could not mount your keiki with sphag but I have never done it yet.
To mount bare root keikis I normally use green plastic coated wire. This allows me to loosely tie the plant on. Using fishing line can cut into the plant when it is pulled tight (sphag protects the roots when using fishing line.) The wire can be tightened until it is just snug without cutting.
Watering can be done in many ways. You can hit it with a hose if it is outdoors, under a sink inside, or with a spray bottle if you only have 1 or 2 since a spray bottle gets difficult with a lot of plants. Dunking the whole plant works just fine since the leaves dry so fast after being taken out of the bucket.
Bare root it will need water at least ever 2-3 days even daily. With sphag I find 2-3 times a week to be good. Overwatering is not a problem with mounted orchids since there is no pot to hold excess water. Sphag will also turn hard white when overly dry and soft and brown when wet. It is much easier to judge water when mounted.