WARNING: this is not mine i found this on maker world and simply decided to share

For a while now, as a plus-size person, I've been frustrated with the lack of options available for belt extensions for DX Kamen Rider belts. The only market options available are:

Flimsy, limited plastic extenders
And expensive CSM-style replacement belt parts
And frankly, both of these suck. They're not compatible with every belt, don't always work with belt attachments/holders, are overpriced, and require multiple purchases if you want one for every belt in your collection to ensure they match the DX belt's colorway/design.

My aim was to fix all of these issues with this design; to create a simple-to-print, easy to assemble extension belt option, compatible with any standard DX/Legend belt, and requiring only minimal, low-cost materials to do so so long as you have a 3D printer.

And big thanks to chairhome on Thingiverse. I used his model as a starting point which saved me a bunch of time recreating the DX belt notches. You can find his model here: https://www.thingiverse.com/thing:6687734

Build Instructions:
To assemble an extension belt youself, you'll require 4 things in total:

Each part of the included model file
A standard width luggage strap (5 cm/2 inch)
Scissors
A lighter
A pen
Print each part of the included model

Grab your luggage strap and prepare to make your first cut. Identify the side of the clasp without the adjustable release, and cut about 4-5 inches from the edge of the clasp to give youself some space to work with.

After making the cut, use a lighter to lightly singe the cut edge to prevent future fraying. (For your safety, be careful to not hold the lighter to close to the edge and to apply heat for only a few seconds at most.)

Grab one of your flat extension pieces and line up your cut edge like so. Holding the strap in place, flip the piece over and, using the holes as a guide, use a pen to mark cutting guides the strap.

Using your scissors (additional tools like a hobby knife may help here), cut both holes out as cleanly as possible. A cleaner finish will ensure an easier fit of the pieces in the next step.

To finish this side of the extension, first take your belt and insert the holes through the pegs of one of the smaller pieces. Then take a flat piece and push it until it clicks into place.

Before we cut the longer side of the extension, me must first reduce the maximum length of the remaining strap. To do this, first readjust the length of the strap like so, with the adjustment buckle right up against the clasp, and the entire length of the strap on the underside.

Connect both clasps together and, using one of your own DX belts as reference, hold the clasp we just finished where it would naturally llie on your waist. Using your fingers, identify where on the other side the cut should be made to fit your waist.

Add a few extra inches to create a bit of slack and ensure that your belt won't be too short before cutting. You can always adjust the strap afterwards, but this prevents you from accidentally cutting off too much. Then continue to repeat steps 2 through 4, using the remaining 2 printed pieces to finish constructing your extension. Now, simply adjust the strap until the fit is comfortable with whichever driver you use it for.

Update: 18/04/25

Adjusted the length of the male connector to make it compatible with earlier DX belts and their remolds (e.g. the Decadriver & Legendriver), of which the length of the female connection point is measured at 26mm rather than today's standard of 28mm.

Have also introduced a number of bevels on many of the model's edges to improve comfort when using the extension for long periods of time.