I suspect that it would fit a 300D. Look around the battery door for a small sliding tab that accommodates the power cable. If there is one, it should fit. Then it would just be a case of supplying it with around 8V DC. Several sites I just checked said that the DR-400 will fit the 300D.
I had a go at removing the casing from the old BP-511 clones, but they are on there fairly well. I will fully discharge those batteries and then have a better go. I got too distracted playing with my new green laser pointer in the smoke from the fires on North Stradbroke Island to try too hard tonight
Eric