Instructions by Nicola Zee.
Purchase 5mm thick black foam board. Size A3 (420 x 297mm). A box of 5 or 10 sheets is not expensive and means you can afford to make mistakes!
On a flat solid surface which you don't mind damaging, carefully
use a sharp large craft knife to cut a square shape 272mm by 272mm
from a sheet.
Use the L shapes to check the size of the square is correct.
If the size cut is slightly too large that won't be a big issue as the L shapes don't have to be glued closely together.
If the size cut is noticeably smaller, then start again and cut a new square.
Cut 4 pieces of foam board offcuts around 230mm long.
The exact length does not matter but the tray will look better if all 4 pieces have the same length.
The height is important and must be 5mm higher than the L shape height.
So, if the L shapes are 25mm high, the 4 offcuts of foam board must be 30mm high.
Check all 4 offcuts match each other in both height and approximate length.
Glue Offcut 1 to a corner of the square flush with the end of the
square and with the bottom of the offcut flush with the bottom of the
Glue Offcut 2 to the same corner flush with Offcut 2 to complete a corner shape.
Glue Offcut 3 so it is opposite to Offset 1
Glue Offcut 4 so it is opposite to Offset 2
One the glue has fully set, check the 2 L shapes sit in the middle
OK and the appropriate tiles fit where they should. If not start
Glue the 2 L shapes ensuring you place them in the correct place.