Depron is a proprietary insulation foam that comes in various thicknesses from 6mm and smaller: it is very light and resilient, easy to cut and glue and takes paint well. It is not particularly strong but can be skinned with tissue or light glass cloth if required.
I’ve made a number of depron models at around the 4 - 5’ size and because it is quick and easy to work a month in the shed can end with a light realistic aircraft that flies well. Much of the look of them is enhanced by vinyl decals from Tim Calvert at modelmarkings.com.
For power I use 20g 1500 kv “”pole and Stator” motors from Robotbirds (also known as “Blue Wonders), with 10 or 12 A SC and 2 or 3 cells for 7 x 6 or 6 x 4 props.