I actually built the carrier in 2005. I was out back cleaning the area behind the shop this morning and found it. I had to take a dirt bike to Camp Pendleton to train some marines and didn't want to tow a trailer in CA because I didn't feel like being stuck in the right lane at 55mph.
I actually copied it from one that my friend had in his retail shop. I think it retailed for around $200 and had a weight capacity of 400 lbs
This carrier is a 2" square going into the receiver.
I have a friend that carries his Triumph Thruxton on the back of his TDI golf in much the same way. I'll email him and see what carrier he uses
Any decent fab shop should be able to build one suitable for the Grom for around $200, and you can match it to the hitch you get which needn't be very heavy duty to transport a 225lb bike