Roblox: see it on an avatar (free)
Put your exported shirt on a test body in Roblox Studio. No fees, no store uploads.
Video coming soon. The written walkthrough below is the whole build.
Written walkthrough
Put your exported shirt on a test body in Roblox Studio. You need a computer with Roblox Studio (free download). The plugin method needs no account; keeping the shirt after the session needs a Roblox account (free). No Robux needed.
1. Export the Roblox PNG
In Ünflat, open the Share menu and tap For a game: Roblox, .glb, sprites…, then Roblox. Tap Download .png. Move the file to your computer (the share sheet, a drive, or an email to yourself).
2. Put it on a rig with the plugin
Open a new place in Studio. In the Plugins tab, tap the Ünflat button, then Dress a rig… and select the .png. The plugin puts it on the selected rig, or on a new block rig when nothing is selected. Nothing is uploaded and no account is needed. Classic clothing is made for blocky avatars, so the block rig shows the shirt as designed.
The preview lasts for the Studio session. Close Studio and it is gone. The next two steps keep it.
3. Import the picture to keep it
Drag the PNG into Studio’s Asset Manager (View tab if it is hidden). Studio uploads it to your account as an image, free. When it appears, right-click it and Copy asset ID.
4. Put the saved shirt on the rig
In the Explorer, hover the rig, press the +, and insert a Shirt (or Pants, or ShirtGraphic for a t-shirt). Select it and paste the asset ID into its ShirtTemplate property (PantsTemplate for pants, Graphic for t-shirts). Studio fixes the ID format on paste, and the shirt appears on the body. A shirt the plugin dressed already has the property: paste the id over it.
Orbit the camera to check the shirt from every side. Selling it on the Marketplace is a separate walkthrough with real fees.