Get WeatherTech all weather mats. The ones in the back of my RX-350 curve a bit on the ends so no damage is done.

I use the heavy metal tent signs, and some plastic and metal tent signs (the heavy ones are good for our high-wind days.)  I drive a crossover, but I don't put them in the back.  I put the folded signs, standing up, behind the driver or passenger seat.  I made Polarfleece "sleeves" (like a pillowcase, closed on three sides) to slip over the signs to keep them from getting scratches.  I don't always use them, now that my signs are more beat up.

I do have mats in the back seat, so that even if the sign legs have some soil or snow on them, it won't mess up my car's carpet.

I have sandwhich signs as well as in ground signs - they go in my trunk and all is well

That's why I don't want those things in my car. You're lucky it didn't do extensive damage as you say 'blemish'. Years ago my regular sign poked a whole outward about the rear wheel. Not funny.

I put mine in the back seat now & all is well.

we use the tent signs and they fit in everyone's trunk....

Don't buy new signs - just put them on a quilt or cover the edges with something soft that won't allow the signs to scrape your interior.

Bungee cords or bungee cord netting.

Take it to the trunk of the car. I put a towel in the trunk of my car, and all it requires is a little shake of the towel after the OH, and all is well.

Several excellent solutions. I just 'borrow' my husband's truck to haul signs & anything that I didn't want in my Cadillac.

Twice a year I clean the inside of his truck as 'thank you'. (If I don't clean his truck it does not get done. On the flip-side I used to 'borrow ' my car when he had important clients. He is now retired.)

Consider some vinyl signs that you can roll up on a couple stakes. https://www.signarama.com/banners/custom-vinyl-banners

Don't you just hate that? I upvoted Joe Pryor's answer. Protect your interior, whatever type of sign you use.

Slow down!

Lots of signs fit into the boot of my SUV.  Movement is constrained by wedging towels carried in the event boots need drying or foot prints wiped after a rain.

The rubber cargo mat in the boot protects the ride along with the convenient loops to which bungee cords can be attached.

Finally, I use the material often placed under throw rugs to prevent slippage between the signs I don't want marred.

In my case I doubt of any of those measures are needed to prevent movement. I do it to eliminate projectiles in the event of an accident.

You could give the dimensions of the sign to an awning maker and they could make a custom sleeve for you to protect your cars interior Esther Preston .

You have received some very good suggestions.

Debe Maxwell, CRS nailed this one!

I've seen those plastic sandwich boards before. A little search on line may pay-off.

Hi Esther -- I use sandwich signs as well.  I stack them carefully in the trunk area and don't typically have an issue.  My vehicle is smaller than yours -- maybe that makes a difference.  

In our area there is a service that will deliver open house directional signs and pick them up.  Many use the service but I kinda like the exercise involved in the early mornings --- you never know who you will meet.  

Try blankets between the signs.

I just have blankets in my hood and leave it between signs. 

I have the same car.  I'd suggest using two of the four tie down hooks (two in the front are where the cargo net clips into) and get a piece of rope to hold them in place.  Maybe go diagonal across from the back to the front.

I'd hire a service to go out and place your signs at 9:00am in the morning. Put out a dozen or so. Put an Open Sunday, 1 to 4pm rider on your sign. EZ Peasey! Many new home communities use these folks.

  Conversely, make up a big "Sandwhich" sign with straps and wear it and walk the neighborhood before hand or better yet take a walk down main street of your town! Even more fun? Wear flesh colored tights! Talk about publicity! Yowsa!

You have many answers here.

I'm a real estate broker and if anyone gets upset because a  little dirt might be in my car from a sign they need to find a new agent.  If it bothered me, I'd get a different job.  I have rubber floor mats and in the the hatch of my SUV.  It wipes, vacuums right up.  And I get my car interior cleaned about once a month.

No easy solution. Sandwich boards are popular in my current nelghborhood

I just throw mine in my trunk and don't worry about it.

I use old beach towels between signs; I drive the same car.

I also use a rolled up blanket to prevent the side to side gap.

Sorry, no... I don't do open houses.

I like Joe Pryor's answer 

I use something similar that the frame opens up and the sign swings with the breeze.

I place a blue tarp sign over. The kind you put over the roof.  I place signs and carry paint tools use it as a truck inside a SUV. I place a few towels as dampers.  The fix should not cost more than $10.

Beach towels for wrappers.

Esther,

It depends when your market is...

I have a lincoln MKS and open house signs fit good in back.

No idea.

Esther Preston - an excellent suggestion from Joe Pryor.

Have the same car, and put them in the back, so no damage is done to the interior.

I wrap my signs (sandwhich) in between large beach towels and/or a small blanket.