An couple attempts were made to use corrugated cardboard as the construction material, but it proved to be not rigid or strong enough. In the end 3D printed components were used. STL files are below. You should use sparse infill (~10%) on all parts to minimize the mass of the finished bot. It probably doesn't do much to offset the large battery, but it doesn't hurt.
You will need to print 2 legs, 2 motor arms, 4 plain arms, 10 joint caps, and one each of the body and tray pieces.
The CRICKIT and battery trays also work as structural elements to attach the sides together, and are based on the mount in this guide. As such it can accept threaded brass M3 inserts. If you use inserts, only put them in the CRICKIT tray. Note that the hole spacing on the two trays isn't identical so you will need to print the two files and keep them sorted out.
Not to worry! You can use a 3D printing service such as 3DHubs or MakeXYZ to have a local 3D printer operator 3D print and ship you parts to you. This is a great way to get your parts 3D printed by local makers. You could also try checking out your local Library or search for a Maker Space.
Solder an approximatey 15 cm (6") wire to each motor connection if there isn't any yet. Strip and tin the other ends.
Snap a motor into each side piece and secure with the 25mm M3 bolts. Side the bolts through the motor mounting holes from the outside of the body pieces. Add a washer and hex-nut to each and tighten the bolts being careful not to overtighten.
Having the motor nestle into the side panel like this spreads out the rotational force of moving the large legs. These are high torque motors and work pretty hard. By having the motors suck into the side plates, the bolts aren't subjected to all the force, in fact they mostly serve to hold the motors in place.
Mount the battery you are using on the battery tray.
A couple heavy rubber bands can be used to secure the battery in place.
If you use a large LiPo (e.g. the 4400 or 6600 mAh versions) and the boost converter, the converter can be secured under the battery mounting tray. You can add a screw terminal connector to the 5v output. Then you can use one of the products below to connect to the CRICKIT.
Add threaded inserts to the CRICKIT mounting tray.
Mount your controller board onto the CRICKIT now and mount the CRICKIT on the tray.
Now you can start putting the bot together. First you can secure the CRICKIT mount to one of the side pieces with a couple of the 20mm bolts.
Next, wire the motors to the CRICKIT. Do a quick test (using the actual software is a good approach) to verify that the motors are wired the right way around. Fix it if the connections are backward.
Finally, secure the other side to the CRICKIT tray with two more of the 20mm bolts.
Place the motor arms over the motor drive shafts. The holes will likely need to be cleaned up somewhat; it should be a tight fit but shouldn't require excessive force.
The other legs simply have an end peg inserted into one of the other holes of each side piece. They should rotate freely and shouldn't need any cleanup.
Secure each non-motor leg with a joint cap and a M3x6mm bolt.
Once all legs are secured, the battery tray can be mounted in the same way the CRICKIT tray was. Mount it so that the wiring to the CRICKIT is the most direct.
Slip a legs onto the pegs of the three arms on one side. Try not to move the motor arms much when doing this. Secure all three leg joints with a joint cap. Do the same for the other side.