What is going to be your main game of choice? Is it fighting games? (only saying this based on your sketchup drawings).
If so, id re-think the u360's. Large amount of throw, weak springback (even with upgraded springs) and funnily enough - the lack of clicking (as there are no switches) is oddly disorientating.
I tend to second this one.. if fighting games are going to be the main choice, perhaps a servo-stik will be a better choice.. the 4-way mode is superior to the u360 with the octagonal restrictor (and just a tad better than a U360 with top mount restrictor in 4-way mode) IMHO, and for fighting games I think a "clicky" 8 way with a short shaft is better. It is a feedback thing for me...
And YES, I have the U360's in my cab with the top mount restrictors, longer shafts, and the stiffer springs (that come with the kit Ultimarc sells). I ALSO did a build with the octagonal restrictor, longer shafts, and heavy springs (again all ordered from Ultimarc and yes the octagonal was the new design) and while it is pretty nice as an overall stick, it is still a pretty long throw and not quite as good in 4 way games. I also did a build with the servo stiks and I have 2 j-stiks in my cab as well, so I have an opinion of them based on experience as well.
However, joysticks are VERY much a "preference" item, and while one person might say that you need to go one way, it really comes down to what you like. The U360 is a great all around stick that gives probably the best experience one stick can give if you are going to play all games equally and have to limit yourself to one stick. It certainly doesn't suck for fighting games, it just isn't the best choice (again, IMHO). Any way you look at it though, not only have I read it a thousand times here, I have also said it more than once, figure out what games you want to play and then build it around that.
To the OP, I like the design, I hope you can pull it off.