Perfectly good flying models with flying surfaces made from 2mm wall foam can be built.
The foam is that used for decorating purposes and sold in large rolls similar to wallpaper
Living Room Stick (7" wings comfortably.
Wilco food bag special
Covering materials need not be the expensive 'Mylar' if you only need a Sports Hall performance. The model here is covered with Wilco Economy Food Bag material, stuck with Pritt Stick
Proper indoor modelling
In 2008 the BMFA promoted competitions for lightweight versions of their beginners model the 'Gyminnie Cricket and I took up the challenge and have competed several times, eventually winning the class at the 2010 indoor nationals .