Shape effects

Well, this is an nontraditional solution.  Many UNICODE and Emoji characters work as input and that includes ones that are shapes. For example, this random string of UNICODE characters:

🔴❌▊ ◣ ➜ ▓ ▣ 🂥

[After I posted this, some characters were not visible, so this is an image: . Not all UNICODE characters are visible in all platforms, browsers, apps, OS's, etc. Most important here is whether Text Effects recognises them.]

Results in (with a prompt of "marbles"):

So, it cannot be an arbitrary shape, but there are many squares, circles, triangles, etc as UNICODE characters.

The above characters are:

• 🔴LARGE RED CIRCLE. [U+1F534]
• ❌ CROSS MARK, [U+274C]
• ▊LEFT THREE QUARTERS BLOCK, [U+258A]
• ◣ BLACK LOWER LEFT TRIANGLE, [U+25E3]
• ➜ HEAVY ROUND-TIPPED RIGHTWARDS ARROW, [U+279C]
• ▓ DARK SHADE, [U+2593]
• ▣ WHITE SQUARE CONTAINING BLACK SMALL SQUARE, [U+25A3]
• 🂥 PLAYING CARD FIVE OF SPADES, [U+1F0A5]

For tables of all UNICODE characters, please see the UNICODE website: Unicode 15.1 Character Code Charts