Hi Bangarang,
Thanks and welcome to BYOAC. To answer your questions:
1) Yes, you could just substitute the curved top with an angled straight board. That would be an easy substitution as you are right the sides and top of the marquee would hide it pretty easily...unless you are well over 180cm(6')!
In fact, Nacimroc built a cab using similar curves and used a straight board to do his top:
Nacimroc2) I cut the holes on the sides using a circle jig I built. The diameter is 15" which also happens to be the inside diameter of the neon speaker rings. I did the cutting first, before I had ordered the rings but fortunately the 15" diameter worked out fine for the 15" rings (again inside diameter).
As for the speaker sandwich I didn't go into great detail on it because it was Knievel's technique that he used on his Neon MAME cabinet which I "borrowed"
If you look up his Neon Mame on the Knievel Customs website he details the sandwich but I will repeat here as I pretty much used his formula with just minor modification.
Knievel's Speaker SandwhichBasically you cut out a piece of plexi glass that covers the inside of the circle. I used a 16" x 16" square cut of plexi glass. You place this plexi behind the neon speaker ring. The next layer is your artwork. The artwork is also 16" x 16". The 3rd layer is a piece of hardboard that I also cut 16" x 16". You then screw all the layers through the screw holes on the neon speaker ring into the inside of the cab.
The only thing I will be modifying is that I will be adding a small slit into the back of the hardboard to backlight the crevasse. I got lazy this past long weekend and only played..no building
3) You could likely get by connecting all the wiring to one PC Power Supply. However, I am using an LCD strip in the marquee for lighting (it has a molex connector 12v) as well plus both coin inserts have red LED lights with molex connectors (12v). Ill take some pictures but basically I also have the neon rings which are 12v and then the power requirements for the LEDWiz (5v). To accomodate all these power requirements I added a 2nd power supply and shorted the green wire in position 4 to the black ground next to it. This allows you to power your power supply without being plugged into a mother board.
End result is that I have a dedicated power supply for all the cab's lighting and the main PC just handles the USB feeds and video/cpu.
I can create a small tutorial if it would help but it wasn't that hard to connect them to the Power supply. Use a terminal strip as it simplifies what you need to do...
For example both the LEDWiz and Neon Speaker rings have two wire requirements, power and ground. On the Neon speaker rings the power and ground were a red and black cable respectively. However, the red unlike the red on a PC power supply was 12v...the red on a pc PS is 5v and the yellow is 12v. So I wanted to tap the red 12v of neon speaker ring into the yellow 12v of the PC PS. Using a Terminal Strip I cut off a molex connector from one of the PC PS wires. Taped up the 5v and its ground and ran the 12v and its ground into the terminal strip. On the other side of the terminal strip I connected the Neon Speaker ring wires (12v joining 12v and ground joining ground yellow pc to red neon and black to black).
For the LEDWiz I did the same thing only this time 5v to 5v (red to red and black to black)
4) Again, for volume I borrowed liberally, this time from Martijn's excellent SF cab. I used a 2.1 Logitech Z313 set. I just extended the volume and headphone as he did (although I haven't mounted them permanently inside the buttons yet. I didn't hack anything just extended what was already there as the volume dial fits in the button np as does the headphone jack.
Martijn's SF Cab