I top mounted my U360s on 3/4" MDF so that I wouldn't need the longer shafts. Even then, the sticks are a bit shorter then my top mounted Happ Comps (P3 & 4). I probably should have used Sanwa for P3 & 4 so that the sticks & bats would match.
I use the bat tops & circular restrictor which sometimes isn't as good as a 8-way, but it is fine for me.
You really do need the stiffer springs. I ordered them with the sticks because everyone else seemed to think they were necessary and I didn't want to pay extra shipping later just for springs. For the most part I thought everyone was just being difficult and the soft springs would be fine. I was wrong. With the soft springs it seems like a strong wind would move the stick. It is unlike any other stick I have used. I'm glad I ordered the stiffer springs.
The top of the U360 is sheet metal so it is basically flat except for the holes in the metal. I'm not done with the CPO yet, but I will eventually need to countersink the flat head screws I am using, and then fill in (bondo) all of the holes before adding the overlay. Bottom mounting would eliminate the need for this bondo step, but then you must have the longer shafts. Like I said, the shafts are already about 1/2" shorter than Happ Comps to begin with. You can't lose another 3/4".
I don't know anything about a joystick mounting kit. Mounting the U360's couldn't have been easier. For bottom mount, just drill out the hole for the stick, and then screw the mount into the bottom of the control panel. For top mount, router out a rectangle the size of the top plate, then jigsaw a big hole to make room for the circuit board and the bottom of the stick and then screw into the top of the control panel. There are 8 holes on the top in the sheet metal. Just pick 4 of them and use a wood screw or a screw + nut like I used.