Just planning the details for our trip to Northern Spain this Summer. Apparently, just a UK/Union flag motif on the number plate is not sufficient for driving in Spain, a separate UK sticker is also required.
I have a magnetic one but the boot on our ND is aluminium and the valance below is plastic so no use. I don’t like self adhesive stickers anywhere on the paintwork. I could place one on the rear window glass but of course it will be out of sight with the roof down. I could also place one on the wind deflector as well for when the roof is down. An added complication is the RevoRack on the boot might obscure both. I could fabricate a plate to fix to the rack somehow & place the sticker on that for when the rack is on the boot.
Anyone have any other suggestions?
I know you’ve said you don’t like adhesive stickers on the bodywork. My compromise would be a small UK sticker on the bumper. At a position of your choosing.
I use these ones:
Just spent many times Madges worth on a complete respray and like you I do not have any intention of putting stickers on her so have elected to get some custom paintwork done by a chap called Neil down in Ashford that will include the ‘TOW’ signs for competition, and the UK sign for foreign travel. Work should take place in 3rd week of April, will post pic here if you are still looking.
This suggestion :~
https://www.halfords.com/motoring/car-accessories/exterior-styling/halfords-oval-uk-cling-sticker-543838.html?cm_mmc=Google+PLA-_-Motoring>Motoring+Products>Car+Accessories>Exterior+Styling--Motoring>Motoring+Products>Car+Accessories>Exterior+Styling--543838&_$ja=tsid:|cid:18137721515|agid:|tid:|crid:|nw:x|rnd:3665221723551256482|dvc:c|adp:|mt:|loc:9045782&gad_source=1&gclid=CjwKCAjwt-OwBhBnEiwAgwzrUlZKiKkujjPAsw7FVrMHRUaSb1ZijwiQ0531zqdhIS9FEkVwaq725hoCVLQQAvD_BwE
appears to have been obliviated / misplaced. 
We just put new number plates on the car with the UK bit on it. Quite acceptable
Not acceptable in Spain according to original post?
Get an adhesive one and either use a hairdryer when you get home to remove it, or stick it to your jumper before affixing to the car to make it less sticky.
And enjoy your trip.
Just smear a bit of vaseline on the sticker before application. It will stick fine and be very easy to remove when needed.
Could you obtain a strong magnet which you could place inside your boot or rear bumper, and which would keep your magnetic one in place?
Or sell the ND, buy an NC and use your existing magnetic one? 
So have I, but not 100% sure it’s legal in Spain as you also need a separate UK sticker. 
From GOV site.
“If you’re in Spain, Cyprus or Malta, you must display a UK sticker no matter what is on your number plate”.
That’s just confusing! But whatever works for you 
Also not technically sufficient for travel to Spain either, as per the original question. I’m sure it would be fine, but my usual approach when driving abroad is to not give them a reason to stop you… 
We are doing a road trip around Spain and have to get the white UK sticker, the rules are quite clear aren’t they
Yep, we off in September for 3 weeks.
The magnetic one for me.
Can’t wait. 
June/July for 3 weeks for us - but 2 more holidays before then
Nowhere to stick the magnetic one- won’t stick to plastic bumper or aluminium boot lid.
Self-adhesive (Halfords) worked ok, removed as soon as we got back.
The boot skin on the 3.75 PRHT is STEEL so a magnetic one will and does stick.
I’ve no idea if the magnet idea would actually work or not but you have to admire the logic!
Ah, I didn’t know that!
That’s you sorted then.
I confess I was concerned about getting the stick-on off cleanly, but it left no residue.
Magnetic would have been my preference, but alas, not on the soft-top.
Enjoy your trip
DC
Unfortunately that won’t do for Spain. They insist on the white & black sticker