In guilding a simple body, we can just attach the servo motors back to back. This is done using three or four 2.5" / 6.5 mm long screws. You will need 3 nuts for each one so that the head and one nut grips one servo and other two nuts grip the other.
Alternatively you can cut two pieces of wood. Nominal measurements are 2 inches / 57 mm ling by 0.75" / 20 mm by 0.25 inch / 8 mm tall. You may want to measure the space between servos yourself to ensure a snug fit. You can also look to 3D print an appropriate piece. Measuring carefully, possibly with a caliper, is recommended if you design a custom piece. Align to the top and bottom of the servos where the mounting holes are and secure with wood or self tapping screws.
The secured servos serve as a rigid body for mounting other parts including a mini breadboard on one side and a LiPo battery and Circuit Playground on the other.
I designed the robot to sit pretty close to the ground but this is a design decision you can change without any consequences. I could not source wheels approximately 100 mm in diameter with a Futaba/Parallax servo mount, so I made my own. You can source wheels if you like from a shop specializing in robotics.
Using good stiff cardboard, I made circles with a diameter of 3.75 inches / 95 mm. You can find a can or other round shape at least this size to trace around. You can also draw a circle with a compass. Be sure the circle is nice and round when you cut it out as any flat spots will not allow the robot to roll well. Cut out two circles.
To find the center of the circle, I suggest following this tutorial on instructables.com or you may Google "Find center of a circle". Using a servo screw to carefully poke through and make a hole at the center point only large enough for the screw. If you have some tiny lock washers, they can help keep the wheels screws tight. If the robot is not moving when the servos spin, check the wheel center screws to ensure they are tight.
As this is an educational build, I did not cut the generous length of servo cable on each motor. Rather the excess is folded and secured with a wire tie and tucked it onto the top of the servo.
While technically these servos require 5V power to run, you can get away with running them from 4V power like the LiPo, and they'll work OK! They'll just run a little weak/slow.
Leave enough slack at the end to just touch the mini breadboard. The ends of the servo cables will be mounting to one side of the breadboard.