SOME designs impliment the washout by the LE being drooped... others by the TE being raised. If the same angle is achieved at root and tip... there is little difference in most cases.

The typical .40 size trainer would impliment the washout at the TE. (as with the GP PT-40 and PT-60 kits)

if you have not added the fixed TE section and its outboard of the ailerons... you can reflex the TE up and add washout very effectively.

Chopping the wing skin and foam core at a diagonal could compromise the structure of the wing... I would avoid doing that if I could.

Decreasing wing loading decreases landing speed... and adding on to the wingspan is more effective for this than adding chord (higher aspect ratio is more efficient for the same wing area.) putting the washout into the added section would be reasonably effective.

Another way to help the center stall before the tip is to make the LE sharper near the center of span. This is usually a simple thing to do... Shave about 1/2 span of the LE flat... and CA on 1/4 inch triangle stock (hypotenuse to the flattened LE) then very slightly round it... and blend the last inch or two to the original LE shape. The center having a sharper LE makes the AOA more critical, and you get a sudden stall at center span with a delayed stall at outer due to the more rounded LE section.

There are many ways to delay or accelerate when a portion of an airfoil stalls... washout is just one.