Sorry this has taken so long to post but real life interfered then last night I got working on the blog and discovered I was missing some pictures then the camera battery died and I had to wait for it to charge.
I used my utility knife for the cuts and my X-Acto knife to separate the foam from the cardboard.
Take the 20 x 30 sheet of doubled foamcore and cut it half lengthwise so you have two pieces 30 x 10.
Take the second sheet which has one half double foamcore and the other half single foamcore. Cut along the double sheet to separate the single sheet from it. Set the piece of single sheet aside for now.
On each of your 30 x 10 pieces of double foamcore, mark two vertical lines - one at the 9 5/8" mark and another at the 10" mark. Very carefully cut along each line trying to cut only through the top two layers of cardboard and foam but not the very bottom layer of paper. If you do cut through a bit of the bottom paper, it's not an insurmountable problem as you will be gluing everything together eventually but it will be tidier if you don't have to deal with it.
Fold the foamcore back against itself at the 10" mark. This will allow you take your X-acto knife and carefully cut along the back of the 3/8" wide piece to remove it leaving the final piece of backing in place as below.
At this point I realized I hadn't taken pictures to show this so I took a scrap of foamcore from a previous project to show this step. AND THEN I HAD MY 'AHA' MOMENT!
I made my vertical cuts just through the middle layer of cardboard. When I folded the right hand side backward, I heard a pop and the foam that hadn't been cut through broke right along the cut line! So no worry about accidently cutting through the back cardboard. Wish this had happened earlier. My other cuts would have been so much easier.
Here you can see the 3/8" section that needs to be removed.
I used my X-Acto knife to cut under the foam to begin removing it.
The top layer of foam and cardboard just peeled off.
Then the bottom layer of foam peeled off fairly easily with a bit of encouragement from the X-Acto blade.
Here you can see how the foamcore folds into the hole to create your right angle.
First sheet done...
Once both sheets have the vertical cut out, you need to draw a line 3/8" in from the edge of the tops and left hand sides (at the end of the 20" piece) of both pieces. Then cut away all but the back piece of cardboard. (You can just barely see the result at the top of the bottom piece below.
You then fold the 20" piece into the hole you have formed. This will give you a 20" ceiling (or floor) and a 10" wall. When you have done this to both pieces, you will have two "L" shaped pieces which you put together like this.
Because I'm demonstrating this on Saturday, I have not glued it at this point. It is put together with "T" pins.
Place the box face down. Take the remaining 20" x 10" piece of double foamcore and place it carefully within the 3/8" border of cardboard and pin it in place.
You still have the single layer of 10" x 20" foamcore left. There are a few possibilities for this.
You can cut a 2" x 20" piece and put it along the ceiling about 1" from the back wall. You can then backlight your scene by adding a string of LED lights behind that. You can cut a 3/16" channel in the ceiling and side walls and fit your piece into that (after trimming back to 19 5/8" long). This will lend additional stability to the room box.
OR
You can trim the sheet of foamcore to 19 5/8" x 9 5/8"and cut a picture window in it . Place an outdoor scene on the back wall, perhaps add some LED lights then cut a 3/16" channel all the way around the roombox about 1 1/2" or so from the back wall and insert your second back wall.
OR
Use a double layer of foam core and
Build a partial wall between, say, a bedroom and a study. Build in a bookshelf and/or space for a TV.
OR
Build a full wall with a door and make a bedroom with an ensuite.
If you want to use the room without any divider, it would be good for a living room/dining room; kitchen/dining room; or just about any combination you want.
NOTE: This was built to make the optimum use of 3 sheets of foam core. The use of the laminated foam core gives it a stability that a single sheet would would not have. And the foamcore into foamcore joints give it further stability and strength.
The box itself can be any dimensions that you choose....
FURTHER: I will re-visit this after Saturday when I demonstrate it for my Mini club. That will allow me to address any questions they may have, make any adjustments, and actually glue the room together.