I sometimes laminate a smaller negative to a larger one. For example, I'll hit a 4x5 sheet with multiple flash bursts, then develop it. I'll then scrape off part of the emulsion, and plop in a trimmed 35mm neg, or two. I use the laminating sheets from Staples or Office Depot. There are a few examples in my Gallery posts (my earliest images). In my case, I also scratch the larger negative, and selectively bleach/tone the image area(s).
You're only limited by your imagination...