presenting evidence of research to grad school committees I am filling in a form for a Masters application. At the top is a question asking to describe research experience (if any) and interests. It's in an engineering field, if that matters.

I have worked on a research project where something somewhat novel was designed, but unfortunately, did not publish a paper. However, I would still like to mention this and talk about it. Since committees are usually looking for strong evidence of research potential for grad programs, what is the best way to express this outside of a 1-2 paragraph description in the form? Would a simple website suffice? Would people in the committee click a link to an external page?
While a link to a website is probably fine, it might not be followed up, so make your short description clear. A link to a page on a university site or other obviously "safe" domain is preferred, of course. 

And, if you have a CV to submit, you can mention it there as "unpublished undergraduate research". 

But the "best" way is to have one of your letter writers mention it in their LoR. This is especially true if the project supervisor is one of the writers. But another professor who knows of the work can also make a brief mention.