I've suggested this before in this forum.
Everyone operates with different aspirations and different constraints.
Walmart's cheapest pillows are sometimes on sale for $2.50 instead of a regular price of $5. That's all polyster, batting and cover.
The L X W of 4 of them is perfect for the Clark. Arguably, two are a bit undersized for the pair of larger Clark pockets.
IMO, they are all overstuffed for this repurposing, with about 4" of uncompressed loft/spring; where there is room in the pockets, due to the pleats for perhaps 2.5". So, If you are willing to open the pillows, you can reduce the batts. Alternatively, on the cheap and easy, you might find all-polyester pillow cases at a $ store.....for $1 and cannibalize 2-3 of the pillows.
Or just overstuff, and let body weight compress the batts, as your head would if you were using the pillows as pillows.
To be clear: This is all proof of concept, bulky, and on the cheap. The batting is relatively coarse, so I am sure it does not have close to maximum insulating value of quality batting.
But, then, I've described a $15 total expenditure.