Billkatz: Isn't it nice to put the roundovers on the cab? Makes it look like a real cab, at last!

Good stuff.
The large, plastic corners have a 1/2" radius, but 3/8" works very well too. I used 3/8" on all my cabs and you can't tell that they're not a 1/2" radius as the corners fit perfectly. I've found that the 1/2" radius, while looking "this much nicer" can dig down too far and hit countersunk screws or brad nails. Bad for the router bit and bad for you if it tears out a chunk of plywood along with the nail. So 3/8" has worked well for me.
As for the access panel, yes, t-nuts are best, allowing you to remove/replace the cover with no worries about sealing. But really, how often will you remove the panel? And even if you replace a blown driver (which you shouldn't have to do if you limit properly) a few times during the life of the cab, COARSE THREAD drywall screws (I used 1.25" long ones) bite hard in plywood and you can always add more foam weather-stripping if you have trouble getting a seal.
In order to use t-nuts on the access panel, you really need to drill the holes and insert the nuts before construction starts, before you glue the flange strips (or 1-piece flange) into the cab. Sure, you CAN get tnuts in there now, but how to line them up with the bolt holes in the access panel...which you probably haven't drilled yet?
Here's what I did:
1. Before adding weather stripping to the flange, determine the center line on your flange pieces. IIRC, my flange is 1.5" wide, so 3/4" is the center line
2. Mark that center line on your access cover, all the way around. Mark where your screws will go. Use enough screws!
3. Lay the access panel in it's place on the cab, ensuring it is centered in the opening.
4. Hold access panel down firmly. Drill two small pilot holes opposite each other through the access panel and flange. Drive two screws through those two holes. The access panel is now screwed down and won't move. Drill the rest of your pilot holes.
5. Repeat for other cabs
6. Remove access panel. Vacuum out all the sawdust from drilling the holes!
7. Using the drilled pilot holes as a guide put a row of 1/2" weatherstripping on either side of the row of holes. Now when you screw down the cover you have a double seal, on either side of the screws, sealing your driver chamber.
I am OCD about these things, so I take what some might think are "unnecessary steps" but it works for me.
ps
Important! If building more than one cab, it is unlikely that you will drill all the access cover screw holes in the exact same spot on all cabs. I labeled the driver chamber and inside of the access panel with matching numbers. Cab1/Lid1, Cab2/Lid2, etc.