To make this D.I.Y. project even easier, have your timber pre-cut at Bunnings to the width of your bath plus an extra 200mm. When choosing your timber, select pieces that have sharp 90-degree long edges and avoid timber with any blemishes.
Choose the best sides of your timber. Place these face down and apply a liberal amount of glue along one of the long edges. Join it to the other piece of timber making sure they're flush. Clamp them together and let the glue dry.
Use the straight edge to mark a straight line close to one end of the joined pieces of timber. From that line, measure and mark the length of your tray. We're using the width of the bath plus 200mm.
Clamp the timber to the workbench and use your circular saw to cut the timber. A straight edge clamp can help guide your circular saw along the line accurately. Once you've cut one end of the timber, turn the wood around, re-clamp it and cut the other end.
Measure and mark the rebates on the bottom of the tray with a straight line. This distance should take into account the width of your bath. We cut ours 100mm in from each end of the timber.
For safety reasons, unplug the circular saw before carrying out this step. Then set the circular saw for the depth of the rebate, ours was 10mm.
Use the circular saw to make multiple cuts in the timber for the rebate. Repeat this step for the other side then tape around the edges of timber you will be cutting out. This will stop wood chipping off from the edges when you use the router.
Use the belt sander and the coarse 120 grit sandpaper to sand the rebates. Then switch to a finer sandpaper and sand the rebates until you have a smooth finish.