I've seen many recommendations on pinning the firewall. When I first heard about it I had no idea what was involved, and it wasn't until I got a giant scale airplane that I saw the light. I knocked this job off in about fifteen minutes (including time for photos).

Step 1 - Mark the locations for the pins

Using a ruler and eyeballing from diagonal corners of adjacent tenons, I made an X between each.



Next, I used an awl to mark the points for drilling.

Step 2 - Drilling the holes

I saw a recommendation for bamboo skewers as pins - I picked up a package of 100 at a local supermarket. Before I could drill the holes I needed the diameter:



I marked the drill bit at 15mm and drilled the holes.

Step 3 - Prepping the pins and the holes

For the six holes I cut six pins, each about 20mm long. I cut the tip off the skewer first.



I opted for Chair Doctor glue from Lee Valley, designed to swell the wood in the joints.



I little squeeze in each hole did the trick.

Step 4 - Installing the pins

I used a wooden mallet to tap the pins into place.



Once they were in, out came the trusty flush cut saw.



Presently the job was done...



... with skewer to spare.

Summary

A fast, easy and almost free modification. The items involved are listed below; those with an accompanying price I didn't already own and had to purchase.

Parts
- Skewers ($1.99)

Tools
- Pencil
- Ruler
- Awl
- Drill
- 7/64" drill bit
- Mallet
- Flush cut saw

Supplies
- Sandpaper
- Glue