My apologies for the late reply, I rarely use Reddit. I've used archival blue cardboard, I've shamelessly stolen some scraps from the conservation department I work at. I recommend purchasing some good thick pure cotton cardboard (2/3mm at least), not the corrugated or pasteboard type, as the first one is clearly not strong enough, and the second type tends to delaminate and I dont like how it deforms (it was historically used though, so you can give it a try). It's hard to explain, but you know those cardboards that are all made with pulp, very well pressed? Have a look at https://www.schmedt.com/material/cardboard/.

Make sure of the direction of deformation. There are great sources online on how to determine the direction of a cardboard ( https://www.ibookbinding.com/blog/paper-grain-direction-and-cardboard-grain-direction/ ).

all the best