The only outlay was for an offcut of 13mm plywood £2.00, a length of m5 threaded rod £2, and some wing nuts and t nuts @ £1.
as I already had the angle grinder and the flap wheel. I have included dimensioned Sketchup diagrams and these were based on a 115mm grinder and with sufficient depth for 20mm thick wood. You may have to adjust your dimensions but none of this was rocket science.
Step 1: Getting Started
I didn't want the box to be too long as the wood generally comes out at 80 to 100 cm and as it would be fed by hand, it needed to emerge from the end of the box before the end of the wood entered the front of the box. I decided 500mm would be adequate for this purpose.
Once cut to length and width I cut the hole for the wheel with a router, this could easily be done with a jigsaw but I was lucky enough to be able to peg the router in the centre of the circle through a hole in the router mounting plate and the bit tracked out at the right circumference to cut the hole.