Local builders sell foam insulation sheets / blocks. We have hot-wire cuttters .... best scratchbuilders on the planet in Wattflyer - likes of Chellie, Quorneng etc. etc. (sorry if I missed your name - you're all great !)
So how simple can it be ?
I have a 1m square sheet of 2" thick styrene .... Now I reckon a slice of that for a fuselage spine. Say 2 x 2 .... a slot through it for a delta 6mm wing sheet.
Slot in top for 6mm Fin and extension running forward.
Simple front mounted ply motor plate and bell motor.
Prop saver + prop
Cheap old small LiPo
Gash bin ESC
4ch radio doing throttle and elevons.
So that's say ... 25" long, 20" wide, 2" sq. fuselage .....