You don't need to customize or craft. If you're in the US, go to 8020.net, and order the amount of 40 series extrusion you need (measure from desk to floor, make sure you get it right, go short if you have to), have them tap the top and bottom with 1/4" or whatever you want holes so you can attach a top plate, and then order another, shorter beam for the horizontal, have them cuz the holes to attach the beams together in the horizontal, and I'm pretty sure they even sell the hardware for the mount there. Then get a clamp off Amazon, a plate from monstertech, and see if monstertech will sell you a top plate to go with it. Order feet that screw in and are threaded the same as the holes from 8020, should take all of 10 minutes to build and only require an allen key
hey m8 with the right extension combo you should be able to floor mount the Rhino, if it's possible for you all you need is a piece of plywood big enough for your chair to sit on with enough room in front for the Rhino to get bolted too, this wouldn't really take up any/much more room than what your chair takes up now.