You don’t need 1/4" thick.
That board is about 77sqr inch and worst case works out to 2 W/in^2.
With the aluminum tariffs in the states you do not want to buy there and import!
Your best bet is locally from the Metal Supermarkets, check to see if you have a store within traveling distance. They ship too.
At 2W/in^2 I’d be looking at 1/8", probably 6061 alloy.
I’m seeing almost $30CAD for a 10" x 14" x 1/8" sheet on-line
{There’s some DIY involved}
You might be able to repurpose something like an old highway sign etc. if you look about.
Cheers
G