Seriously? Ok...
I work for a manufacturer of conduit bodies (and other fittings), and I can tell you that it is absolutely a violation of an existing UL Listing if a conduit body was modified in a manner as depicted in the picture.
For starters, the sidewalls would probably not support the required pull/bend forces required in the UL standard (UL514A and B) - while maintaining a proper seal with the cover gasket. Secondly, it may not meet the required bend radius requirements for the two raceway attachments in close proximity to each other (i.e. U-Turn). FYI, any product with a UL listing is tested and approved for the application for which it was designed. If this fitting was INTENDED to be field modified, the manufacturer would certainly state that in it's instructions - and therefore would be UL listed with such field modifications. Heck, they might even make it easy by providing cast-in dimples to make sure the hole is drilled in the proper location.
If the manufacturer does not specifically state what modifications may be done (if any), then it is safe to assume the modifications render the product not covered by the UL listing unless you paid for a UL field inspector to visit the job site, and evaluate the application and product modifications.
I know if someone called me and asked if our product was still UL listed after such a modification, I would politely say "No". However, if the customer needed to do this frequently, we would offer to look into what it would cost to investigate if such a modification would affect the ability of the conduit body to meet the specifications required to maintain the UL listing. It might or might not be acceptable.
If you think this is OK, because the UL listing or manufacturer doesn't "tell" you that you can't make the modifications, then what would be the purpose of obtaining a UL listing for ANY product? I feel the burden is on the contractor/installer to quantifiably 'prove' that the modifications have not affected the performance characteristics that enabled the product to pass the UL tests and become listed in the first place. Our company wouldn't pay hundreds of thousands of dollars on specific UL testing services, only to find out that "anything goes" when our product reaches the field. There would be no point.....