Throwing out another option that I've found to be great. I've found that the flat carbon spars - 1x5mm or 1x6mm with lengths of 750mm or 1meter are excellent combined with foamboard. They are only stable in one direction unlike the round/square options, but in wings, you only really need stability in up/down axis. They are extremely easy to just put inside of c-fold foam spars - I glue them directly inside of the c-foldss and it really stiffen things up. I've also used them inside the paper on the leading edge of wings to make them much more durable when hitting trees or in combat. I bought mine at hobby king and they are labeled "Carbon Strip", but they are unfortunately back-ordered there now. Model Plane Foam calls them "Carbon Flats", but you'd have to buy them as part of an order with a box of foam. You almost surely could find something on ebay or aliexpress too. The Flitetest store even has some, although in weird decimal point inch measurements that aren't registering in my brain.