Make fake wood out of foam to lessen the weight you have to hold up. That will give you more options for how to attach. If you search the forums (or google using site:halloweenforum.com) you'll find quite a few examples of boarded up windows.

Command strips are great, and they make outdoor ones as needed. Velcro is an option, and you can get BIG, WIDE sticky back velcro that holds a lot (but this can damage paint). You can also put up painter's tape, and hot glue lightweight stuff to the tape.

 

Here is how I did mine...

 

Kakugori suggested some good ideas both with the foam and the command strips. Command strips make some velcro that will hold up to 16 lbs. of weight they make some for less weight too. I actually used the ones for up to 16 lbs to hold some plastic planters onto the top of my deck railings 2 years ago and they are still on there even after some pretty high winds. Before I used the velcro command strips if you looked at the planters they would fall off the railings.
If you use wood I would use the 16 lb velcro strips if you use foam you could go with the ones for less weight.

 

I have used cardboard in the past, but this year used foam, there not done yet but look great!
I have 3 windows to do.

 

I used foam, spray painted brown, and glued with hot glue gun to weed blocker fabric. It works great with vertical hanging windows; not so much with the horizontal ones.

 

I was thinking about using some vinyl or wood floor planks for boarded-up windows. The weight is a concern. I did not think of using supports that rest on the ground!

 

I agree with other posters: If it's going to be temporary, you could try the foam boards and command strip wall hangers; and some of the ones I've gotten in the past had like a 15 or 20 lbs rating per hook. And, you might be able to get away with using museum putty to keep the foam pieces from being caught in the wind or bumped off the command hooks. I've recently priced velcro for a non-Halloween project and I'm still.not over the sticker shock.

Another idea I thought could work for you, is to mount your faux foam boards to a piece of old window screening, then make a quick 1x1 or 2x2 frame to staple the screening to so you can hang the whole window dressing from the command hooks? I'm always finding used window screen scraps at my Habitat Restore....

Aaaand, depending on your eaves situation... you might have the type that are little pop-in plastic or metal panel series, which should you GENTLY press upwards, will let you access the rafter tails of the roof and the soffit fascia or some kind of wood framing. You could very discreetly staple in or use teeny eye hooks into said exposed rafters or framing. Then, you could use heavy guage fishing line or spiderwire to suspend the weight of the foam and the window dressing, using the command hooks or museum putty to keep the prop snug to the building. That way you can get to a stable hardware anchoring situation with no visible damage to the house... because all of the modifications are minute and inside a space nobody will see. Just tape the fishing line where it will have contact with the soffit panels so they won't fray, or, thread your fishing lines against the house into an existing perforation should that line up. In lower lighting, you'll never see the line. Just hang the whole thing like it's a giant, lightweight, picture frame on a extra long picture wire.

Even if your soffit eaves panels look solid, with a screened vent every so many linear feet, usually over a window, there is an excellent chance they are installed in pop-in sections. Just push up gently and you'll find a seam if there is a panel system. The majority of installers don't screw these panels into the cleat track, but if you see a bunch of screws I wouldn't mess with it because loosening painted over screws can peel the paint.