You do not enchant the cloud as a device. You make a device (or learn a spell) that makes clouds solid. E.g. (assuming you can reach a cloud in the first place):
The Cloud as Solid as Earth, Mu Au (Te) 10
Range: Touch, Duration: Sun, Target: Individual
Makes a cloud as solid as packed earth, leaving all its other properties unmodified.
(Base Level 4, change one element into another, -1 magnitude affects only one property of air, + 1 magnitude Touch, + 2 magnitudes Sun duration - compare with Rain of Stones)
A device with Conc duration (-1 magnitude) that maintains concentration (+5 levels), with a single use/day would also be level 10 (a 1 pawn lesser enchanted device - ideal work for apprentices).
I am still curious about the buoyancy thing. If you place some weight on a (solid, but otherwise normal) cloud, would it sink accordingtoMythicEurope*physics? I'd say yes, though perhaps not very quickly. So you'd need something to make the cloud more buoyant.
Now, you could do it with Rego (the base level for controlling a normal weather phenomenon - the cloud - is 3, brought up to level 15 for Touch Range and Sun Duration, plus an extra magnitude for great strength - the castle on top!), but I'd go again with Muto and snip 5 levels:
The Cloud as Light as Fire (Mu Au (Ig) 10 - same guidelines as above, except Ignem in place of Terram; note that the two spells probably qualify as "similar")
R: Touch D: Sun T: Ind
Gives a cloud the same buoyancy as if it was made of Fire. Fire tends to climb higher than Air so the cloud will tend to climb unless weighted down (note that this is not hot air being lighter than cold air, a modern concept - according to Aristotelian physics the "natural" positions of the elements, to which they all tend, is Earth at the bottom, Water on top of Earth, Air on top of Water, Fire on top of Air).
This does put you into an unstable position - you probably need to carefully manage ballast! That's not too difficult, though:
The areonaut's magical ballast (Cr Te 4)
R: Touch, D: Sun, T: Ind
Creates 10 cubic paces (a few dozen tons) of sand. Have the grogs throw the excess off board.
(Base 1, +1 Touch, +2 Sun).
Of course, you probably want propulsion too. CrAu 4 will create a nice, solid wind blowing in the direction you want for as long as you maintain Concentration (Base 2, +1 Touch, +1 Conc); for longer trips, you'll probably want to boost it up one magnitude to Sun.
Note that this is all apprentice level stuff. The last two spells (ballast and propulsion) could be managed spontaneously without fatigue with only a modicum of study. An experienced wizard might manage even the first two spontaneously without fatigue!