If you are going to use Van's ELT and strobe mounts, intall before you put the skins or at least drill while you can get a good angle.
Laying down, in the tunnel in no way to build.
It was said earlier, this would also be a good time to install the bracket for an autopilot pitch servo.
IMHO, don't bother with a flap position or trim sensor. Just extra weight and complexity. You can look out the window and see where the flaps are and with manual trim, you can feel it through the stick and the position of the knob.
Other things I ordered at this time were an Affordable Panels panel, stick grips, and throttle quadrant.
As someone already said, this would be a good time to order a parking brake and while you are at it, order a bunch of wire, although Van's may not have the best price on it at this time.
From Aircraft Spruce (or other source), you might also wish to order the following:
1. An extra 1/4 pound of each of the more common rivets
2. Assortments of AN bolts, washers, nuts, and cotter pins
3. ssortment of every conceivable size of Adel clamp
4. A boat load of all the different plate nuts ( two lug, one lug, & corner nuts in 6-32 and 8-32)
5. A starter kit of assorted crimp on electrical connectors. (Affordable Panels used to sell a great set, maybe someone else is doing that now. Fabian, we miss you!)
In other words, now is the time to finalize your list of "options" and start ordering them.
Just remember to build the plane you want and not the plane others want you to build!