I usually just use some low tack spray adhesive (the stuff sold for stencil painting to stick the stencils on things temporarily) and glue them right to the foam. Then when I'm done cutting the plans just peel right off. If I want to re-use a plan then I have some "waterproof posterboard" I picked up at walmart a few years back that's great for making re-usable templates. It's actually plastic and works great, I just trace around them with a pencil then cut on the inside of the pencil lines. Unfortunately I haven't seen it at the stores the past two years to get more
However I've recently upgraded to cutting my foam with a homemade CNC machine and a
needle cutter. Which I learned about right here on the FT Forums:
http://forum.flitetest.com/showthread.php?24251-Cutting-foam-sheets-with-a-needle!
I had more issues getting mine up and running than most of the others who've built them but am loving it now. Last night I cut a mini-corsair which I managed to squeeze into one sheet of foam:
That took 40 minutes to cut, and I probably spent another 30-40 minutes preparing the files and generating the gcode. But I can now cut as many more as I want and it only takes the 40 minutes for the actual cutting.
Most of the things I've cut have gone considerably faster - this just has a lot of pieces on one sheet. Most full sheets are 15-30 minutes to cut depending on how many parts are on them. Preparing the files takes 10-20 minutes for most FT designs. The basic procedure is:
Open a new document in Inkscape and set it to 30" wide by 20" tall
Import the PDF - you have to do this one page at a time.
Ungroup the design (sometimes 2-3 times) to get the parts as separate paths.
Delete the text and logos and other stuff I don't need to cut.
Then I either just re-position the parts to fit nice on the 20x30 document - for some designs that's all it takes.
For others, I have to re-select and re-group the individual parts (many of the FT designs the outlines are multiple segments since they leave gaps for holding tabs when they laser cut them.) then move them around to fit them onto the 20x30 document.
On some designs I'll then import a second page of designs onto the same document if there are more parts and I have more room.
Finally I check for parts that need multiples cut and duplicate them.
I then save those as .SVG files for each sheet of foam I'll be cutting. I also put any posterboard parts into a separate SVG which I use to generate gcode for my 2.8w laser diode (another option on the machine) to cut. The laser diode can't cut DTFB, but will make nice clean cuts in posterboard at 300mm/min. I use an inkscape extension from jtech photonics to prepare the laser gcode.
For the foam, I open the .SVG files in estlcam and use that to generate the cutting paths. On some designs it's super quick and the automatic tools in estlcam do most of it for me. But on most of the FT designs those holding tabs mean I have to manually trace the paths - it still assists and auto connects the points so it's not like hand tracing...but is a bit more tedious. I also define my cuts at various depths - I've been using 6mm for full cuts, 2.5mm for score cuts, and 1mm for markings/optional cuts.
It's not really as much work as it sounds like...and gets quicker once you get a bit of practice at it. First time it took me an hour or two to convert a simple plan. The corsair was one of the more complex ones I did and it was only about 40 minutes to prep.
The big expense is building a large CNC...the MPCNC design I used makes that very affordable. 2 rolls of filament for my 3D printer and I spent almost every evening for a month or two printing the parts a couple a day. The hardware for it is very affordable, the rails are just 3/4" EMT conduit from the hardware store which is usually about $1-$2 for 10 feet, to cut foamboard you'll need about a 3'x4' machine so 4 sticks of conduit is still <$10. The designer of the MPCNC sells a
parts bundle with all the electronics, motors, bearings and nuts/screws/washers you need for $245. But I bought my parts here and there as I went to spread out the cost. I may have saved $10-$20 over the bundle price but it was a lot more work...the bundle really is a great deal. (He also sells the printed parts for $190 if you don't have a 3D printer.)
Of course you don't have to use the MPCNC. If you look at the thread I linked you'll see a few people using other CNC machines they've built or bought with the needle cutter. The MPCNC just happens to be a very cost effective way to build a suitable machine.
JasonEricAnderson was working on a low cost CNC design here in our forums as well:
http://forum.flitetest.com/showthread.php?6971-Idea-The-Flyplotter&highlight=flyplotter but he hasn't updated since August so I'm not sure how that's going
So...short answer - low tack spray glue
Long answer - CNC rocks!