I am about to re-frame an exterior bathroom wall which will involve removing an old window and framing in a new 16"h x 4'w glass block window. What I need to know prior to finalizing the rough-in dimensions is what are my modern options for creating a weatherproof wndowsill for this new window. The glass block is a non-mortar construction and the bottom will sit in a PVC track. The other old sills have been covered in aluminum flashing but I don't know if I should do this here (or how to do this).

I would love to use a Plastic, PVC or composite material if available. I need it to be small (3/4" to 1" thick), just enough to run any water away from the window without looking disproportionate to the window size. It will be finished exteriorly with vinyl siding.

1) What is available for non-wood sill options?

2) If nothing, what would you recommend for creating a weatherproof sill for this application?

Thanks.

I would go with the aluminum clad wood. Make a wooden window sill. The usual procedure for covering them is to buy a coil of metal and rent a break. That is how I did it 20 years ago when I covered all my trim. Maybe for just one, go to Lowe's, Home Depot, etc. and buy 1 piece of fascia cover. You should be able to adapt it covering a window sill, at least the top.