Since a giant, steaming and close to being alive jack-o-lantern being hauled by a single truck didn't make much sense already, I thought I could expand the almost fantasy theme even more.
I imagined that rays of light breaking through the pumpkin's shell would look pretty amazing. The light rays exiting a dying ender dragon proved to be excellently suited for this job.

So, the idea was to summon a dying ender dragon inside the pumpkin, let it die and summon a new one as soon as its predecessor has deceased.

To summon a dying ender dragon you can use this command:
/summon minecraft:ender_dragon ~ ~ ~ {Health:0.0f}
However, when the dragon dies it slowly floats upwards and would eventually exit the pumpkin through the roof.
To solve this problem, I changed the command to summon an armor stand with the dragon riding on top of it.
Since the armor stand is stationary and the dragon is "tied" to it because it is riding the armor stand the dragon is stationary, too and perfectly stays in place:
/summon minecraft:armor_stand ~ ~ ~ {Passengers:[{id:"minecraft:ender_dragon",Health:0.0f}],CustomName:"{\"text\":\"EnderDragonSeat\"}"}

I gave the armor stand the custom name "EnderDragonSeat" because we will need to kill the armor stand later when the dragon has died.
If we didn't kill it, there would be a ton of armor stands standing around because we summon a new dragon and armor stand every time after the old dragon has died.
Theoretically, we could simply kill all armor stands in the world but maybe there are some armor stands in other builds that should not be killed.
To be able to select only the armor stands the dragon was riding on, I gave them a custom name no other armor stand in the world should have.

Now, we also would like the armor stand to be invisible because it doesn't look very pleasing. To do this, we only need to insert an Invisible:true tag:
/summon minecraft:armor_stand ~ ~ ~ {Passengers:[{id:"minecraft:ender_dragon",Health:0.0f}],CustomName:"{\"text\":\"EnderDragonSeat\"}",Invisible:true}

In case the armor stand is summonend in midair we don't want it to fall down but to stay where we summoned it, so we give the armor stand a NoGravity:true tag:
/summon minecraft:armor_stand ~ ~ ~ {Passengers:[{id:"minecraft:ender_dragon",Health:0.0f}],CustomName:"{\"text\":\"EnderDragonSeat\"}",Invisible:true,NoGravity:true}

This is the entire command I used for summoning the dragon.
Now there is an invisible armor stand floating somewhere and it would be quite hard to find and destroy it by hand.
But luckily we gave it an explicit name and can use the following command to kill it:
/kill @e[type=minecraft:armor_stand,name=EnderDragonSeat]
With a command to summon a dying ender dragon and one to kill the armor stand it was riding on, we can come back to the original idea.
I put a clock inside the pumpkin that first kills all remaining armor stands called EnderDragonSeat and after that summons a new EnderDragonSeat with a dying ender dragon on top of it. This cycle repeats itself as long as the clock is activated.

The only remaining probem is that optimally we would like to see exclusively the light rays but not the dragon.
Sadly, any potion effects like Invisibility don't have any effect on the ender dragon whether applied manually with splash potions or with commands.

But there is another solution:
As you may have noticed the dragon gradually disappears when dying. There is a texture in the resource pack that defines which pixels disappear first and which ones disappear later using transparency. It is called "dragon_exploding.png".
The more transparent a pixel is, the earlier it disappears.
So, if you make all pixels in the image "dragon_exploding.png" completely transparent or simply erase them (with an image editing program) and put that empty "dragon_exploding.png" back into your resource pack, the dragon already is invisible when being summoned:



For some reason no shader I know can display these light beams well. Most shaders don't show them at all including Sildur's vibrant shaders which I use.
That is why I had to take the last screenshots without shaders, so that you can see the rays of light.