It isn't that hard to include the minibb forum into an existing template, especially so if you use the old minibb design. All I did with my latest forum was to copy the source code of a page of my website that wasn't displaying much content - such as the contact page (only content was an iframe for the contact script hosted on another site). I then took all the code above the iframe and used it inside the 'main_header.html' file above the minibb forum code. Obviously I removed all the minibb code above the actual forum table and moved any minibb code such as meta tags / styling into the new so that it all works as it should. I disabled/altered parts of the forum styling.css so that there were no conflicts with the new styling, as the forum now uses 2 style.css files.
Likewise I did the footer by using all the code below the iframe and altering the main_footer.html so that all the minibb code now resides between the design that the rest of my site uses. Result is here
I don't get any issues with anything and the forum seems to work perfectly.