Door frame gap filler

Ideas please.

I have a combined door and frame (metal door and frame, the frame screws into an existing wooden frame space).

The space for it is almost perfect, but for around 2mm at the side. I need to fill that space to stop the door frame from distorting as I tighten the screws.

I have seen an aluminium strip in B and Q that would do the job. 2m x 40mm x 2mm (almost perfect dimensions for the space). It's a bit overkill though (and a bit pricey).

And other ideas? I haven't seen any woodstrip thin enough or upvc wide enough.

Window packers and you jam in to hold frame in correct place then drill through them. Only screw through where packers are so it don't pull out of shape. 1mm, 2mm, 3mm ect sizes available and other types of packers used for windows, glass, frames

Brilliant. So obvious too. I had my mind fixed on the idea of filling the whole void, but it's really only needed where the screws go. Thank you!