Poster board and tape. Or a hot glue gun works well. Small battens ripped from doorskin and a hot glue gun are my go to for heavier patterns, like bulkheads in a boat. They have the advantage of also seeing if a finished pattern/piece can be taken out in one piece.

I find it quicker and more precise to start with one or two edges and then gluing or taping pieces on to meet the other edges and points. Cutting to fit is slower.

Here's a floor carpet template

Perfect, thanks!