Works in Progress:Recreating it another time in OpenSCAD - models are 'coded', parameters can be changed by modifying some variables easily.Testing with neodymium magnets which would eliminate the need for tape. Turns out it's too complicated and good tape (can be partly peeled & reused) & hard stamping surface is totally fine.- Trying double-sided stamping to further reduce cost by half & make backup smaller.
How to get?To make sure it really works correctly, I'd prefer selling the customized and printed jig.
If I gave the file it could give you headache since I pieced it together quickly and tolerances are quite important when printing it out.. 
But if you're experienced in modifying designs, printing them or are overseas etc. just send me a DM!Get the fresh clean source code below and print it yourself or if interested, I can print for you and ship it to a PO box or something. For best privacy, buying or borrowing a cost-effective printer would be recommended.
[NEW!!] OpenSCAD design finalized!Paste this following code into
OpenSCAD (free, open source) and set the parameters to whatever fits your washers and stamps. It's important to measure the stamp's outside size, not the letter height which is usually given on the product page. Then hit F5 and F6 to preview and render the two models. Finally, export the STL file and print it out.
// n0nce's Steel Washer backup jig
// ---------- configuration ---------- //
washer_dia = 30; // set to your washer's diameter (mm)
washer_hole = 10.5; // set to your washer's hole diameter (mm)
washer_h = 3; // set to your washer's thickness (mm)
stamp_w = 8; // set to your stamp's width (mm)
stamp_d = 8; // set to your stamp's depth (mm)
// ---------- code ---------- //
tolerance = 0.25;
// ---------- outer part ---------- //
module roundedcube(xx, yy, height, radius) {
$fn=60;
translate([0,0,height/2])
hull() {
translate([radius,radius,0])
cylinder(height,radius,radius,true);
translate([xx-radius,radius,0])
cylinder(height,radius,radius,true);
translate([xx-radius,yy-radius,0])
cylinder(height,radius,radius,true);
translate([radius,yy-radius,0])
cylinder(height,radius,radius,true);
}
}
module washer() {
cylinder(h=washer_h, r=washer_dia/2, center=false, $fn=200);
}
module outer() {
module outer_body() {
translate([-27, -27, 0])
roundedcube(54, 54, 20, 3);
}
module slot() {
translate([-5/2,0,washer_h])
cube([5,23,30]);
}
difference(){
outer_body();
// main cylinder to cut out
translate([0, 0, washer_h])
cylinder(h=100, r=38/2, center=false, $fn=200);
// washer cut out
translate([0, 0, -1])
scale([1, 1, 10])
washer();
// number slots cut outs
for (i=[0,2])
rotate(-14+i*14)
slot();
// letter slots cut outs
for (i=[0:7])
rotate(82+i*28)
slot();
}
}
// ---------- inner part ---------- //
module inner() {
module inner_body() {
// main part
cylinder(h=20-washer_h, r=38/2-0.3, center=false, $fn=200);
// handle
cylinder(h=20-washer_h+8, d=9, center=false, $fn=200);
// sticking out part
rotate(-14) translate([-4/2,0,0])
cube([4,22.5,20-washer_h]);
}
module puncher_hole() {
sw=stamp_w+tolerance;
sd=stamp_d+tolerance;
rotate(-14) translate([-sw/2,
(washer_dia+washer_hole)/4-sd/2,
-10])
cube([sw,sd,50]);
}
translate([0, 0, washer_h]) {
difference() {
inner_body();
puncher_hole();
}
}
}
outer();
translate([70,0,0])
inner();
A note on material choice:When you are talking about extreme weather conditions, I don't know if you know this but 304 stainless steel can easily be corrupted if exposed to salt water, so if you are living in area near sea water I would opt out for better 316 stainless steel.
I am not sure if Jameson Lopp did any tests with salt water, but I believe that is much more realistic threat for some parts of the world compared to acid exposure.
Not all stainless steel is made the same, there are different types and grades, so while 316 SS is better for salt water it does have slightly lower melting point compared to 304 SS:
Grade 304 melting points: 1400-1450°C (2552-2642°F)
Grade 316 melting points: 1375-1400°C (2507-2552°F)
Grade 430 melting points: 1425-1510°C (2597-2750°F)
Grade 434 melting points: 1426-1510°C (2600-2750°F)
Grade 410 melting points: 1480-1530°C (2696-2786°F)
Grade 420 melting points: 1450-1510°C (2642-2750°F)
Pick 304 if you want high temperature resistance, or pick 316 if you want high corrosion resistance.
And according to the GIF showing the thing moving, it looks like you just rotate it to hide/reveal the washer? Or does it open/close?
My personal tip though: you don't need to watch videos about everything, sometimes it's better just to try. It can avoid some pain that YouTube videos sometimes induce. Nail polish without glitter can be a helpful tool to touch up a chipped floor tile; just use that little brush and apply, it doesn't get much simpler. The idea about the glitter being at random, sounds good as well, though.
I've spent hours setting up a secure wallet and backups in the past. 
