I’m looking to do some mock ups of parts that are fairly large and flat. iv used cardboard and chipboard but they’re too flimsy. What are you guys using to mock up flat parts? approx 3’x4’
Luan board maybe?
You can always rip up some 1/2" boards and glue it to the back to stiffen things up pretty inexpensively.
What sort of parts are these that you are trying to mock up? a thin fibre glass sheet could do the job easily it just depends what you call inexpensive? fibre glass is very cheap compared to Carbon fibre!
You can try pink board from Home Depot. Or lexan.