I made tiny tweaks, adjusting the shim shape and adding an angle for attacking from the front of the latch. Order sent to SendCutSend. I chose 1075 spring steel, 0.015" (0.4mm). That's rather thin, I hope it works.

I'll upload the SVG Inkscape file to the project files.

Thickness references:

LINE2design Shove Knife: 5/64" (2mm)

SHOVE-R: 1.4mm

credit card: 0.8mm

SendCutSend material options and price estimate:

chromoly 4130
0.050" - 1.3mm
$19.89 /ea
$13.92 /ea @2

spring 1075
0.015" - 0.4mm
$16.47 /ea
$12.85 /ea @2

stainless 304
0.030" - 0.8mm
$12.84 /ea
$8.99 /ea @2

stainless 316
0.060" - 1.5mm
$20.10 /ea
$14.07 /ea @2

titanium grade 2
0.040"  - 1.0mm
$17.21 /ea
$12.05 /ea @2