The main triangle can be flattened to 1.25" (31.5mm) on any planer or wide belt sander that is 24" or wider. You can look for one locally, or if you have no other options, consider making a flatening sled like this one... https://www.google.com/search?q=router+sled+for+pl...

Be sure to balance the part as you sneak up on the 1.25" size. Keep the routed hollows in the center. Watch for twist in the part. This is the time to remove twist. It is at least 1.5" thick. You can take out up to 1/4" of twist if it exists if you plan well.