Adding New Variants: Can One Variant Prompt a Custom Form? (Or Other Ideas for Solutions?)

Site URL: https://www.jessicapoundstone.com

Hello! I sell fine art prints, and currently offer two "customizations" -- size and frame. I currently offer four sizes and four frame styles. I have one variant for each possible combo.

I am getting ready to offer a new frame option: a Lucite plexi box that comes in multiple colors (7 total). What I would like to do is add one variant per size titled "plexi box frame", and then, If a customer chooses the plexi box frame option, have them indicate the color they want via a custom form. Customers would only see this form if they chose the plexi box framing option. Or, alternatively, they would be able to easily dismiss the custom form if they did not choose the plexi box framing option.

The alternative  - having to add multiple colors to every available print - is daunting, so I am hoping the above solution as possible, or there is another solution that could work. 

If it's not possible to avoid the big lift of adding a ton of variants, I will probably only offer it on a few pieces and roll it out slowly, or reduce the number of colors available, both of which would be kind of a bummer.

Thanks in advance for any ideas! 

Relevant URLs on my site:

Product page with multiple products: https://www.jessicapoundstone.com/order-jessica-poundstone-art

Specific product page example: https://www.jessicapoundstone.com/order-jessica-poundstone-art/color-space-27-pink-red-yellow