Electronic – arduino – Which version is the Ethernet Shield

Are you sure that the trailer isn't just there to pad the packet to the appropriate size, or as some form of CRC checksum?

In fact, looking at the RFC it looks to me like a host, which wishes to use trailers, would send the trailer request during ARP, and if the other host understands trailers then it will add it to its list of hosts which understand and use trailers. Therefore in theory it wouldn't be the trailer which was causing your problem. However, I can't categorically rule it out as this is based on my memories of my networking course and a superficial reading of the RFC you linked to and Wikipedia.

You could try one of the freelance design sites like Freelancer. You need to take into account the cost of EMC conformity in the country where you live; CE conformance is required in the EU, and can be quite expensive- something like £3,000 in the UK for that sort of item.