Hi. I'm mech engineer.Just finished similar project for a friend's electronics upgrade on a cc dv t top. No idea if following helps or not. You can build this any way that works for your combo of equip. and existing layout of cross bars of your roof- just need to think it through structurally. Prev poster is absolutely correct in that the loads get high when you mount heavy stuff at this height, but it's not difficult cult to figure out a clean looking ng solution. Bolting can be stronger or weaker than welding and vice versa all depending upon the design. Welding tends to result in a clean OEM look and take less volume, but bolts and plates worked out great for us, and as I said, strength is a matter of design. Aluminum aircraft bodies are riveted not welded for a reason. Fasteners can have a certain degree of give whereas welds are extremely rigid and can crack. Same thing in a bridge, many joints bolted.
As far as starboard or any plastic structural member goes, it depends on the span and if there are existing members to which it will be fastened vs it spanning a large open area. I'd guess that if you are planning the plastic plate to span an open section of more than maybe 12 inch and it has the full load of a dome scanner and metal adapter stantion then I'd go to 3/4 thick. Remember that the stiffness or the amount of deflection increases alot when you make a plate thicker. Twice the thickness results in 8x less deflection, whereas if you double the width of the plate the deflection halves, all assuming the same load. That said, if the plate is under the canvas then the thicker it is the more likely, over time edges and corners will sever the canvas.
My friend bought a short metal angled adapter that sits between the dome and the t top frame. If I remember correctly the lower flange of that adapter had 4 holes which we carefully positioned and thru bolted existing cross members. The adapter sits on top of the canvas and he put silicone caulk between the canvas and the lower flange of the adapter at the locations where he drilled thru the canvas.
You MUST thi nk through any thru holes i n existing members because these will definitely compromise the strength of those members and lead to stress cracks eventually if not done carefully. Use smallest diameter, as few as possible and choose thru location carefully.
Another way to do it in a stronger way would have been to use the same adapter mounted over the canvas but NOT to drill thru cross members, rather fashion an interior plate that sits on the interior ceiling below the existing cross bars and then you clamp the adapter plate to the interior plate and effectively sandwich the existing members thereby avoiding drilling thru.
A third design is to solidly bolt the stb. Plate to the frame where the area of the plate can be cut to find strong mounting bolt locations to the frame and then the scan er and equip is easily bolted to that plate. For example, you run a long plate fully across the top, flexed and then fastened to the two pipes that run fore aft on either side of the top.
This allows mounting of multiple units like a pa, dome and gps unit which can then be situated at the exact place to give a nice look. This means more plate. In my friend's case he ended up mounting gps to the dome adapter unit, so did not really need such a big area plate. One major benefit of this approach is that if done cleverly, you can avoid any holes in the canvas. I avoid popping holes in a continuous barrier membrane like a t top if at all avoidable. Another downside is that the plate is visible since in that approach it sits over the canvas, but if color matches that of roof material it will look just fine in my opinion.
Definitely buy good sst bolts washers etc from a real supplier and figure out exact min adequate length as prev.poster said. A nice finish on interior is to use cap nuts, but to do that you got to really get the length spot on. A few extra washers can help. Life is way too short to go cutting sst. Bolts. It very tough stuff. Hacked protruding long bolts not spaced evenly are the best way to make it look like an amateur low budget aftermarket install. And dangerous if you're ever getting your head up there for maintenance
Hard to advise exactly without seeing your setup, etc. Every situation has unique elements and owner desires to consider.
If u post a pic or provide more details I'm happy to chime in if it's useful.
The other pia for us was making sure we could route the coax cable and wiring inside existing cross tube and then down one of the four uprights. It impacted the exact location and thru bolt locations.
