Bicycle Mechanics - Truck Bed Fork Mounts for Child's Bike?

Hello, I recently purchased a truck bed fork mount, but I need one that fits the space between the forks of a childs bike (the distance is about 3.5", where the one i bought is 4"). Do you know if anyone sells fork mounts in this size? This would really help me out, thanks a lot for anyone's help.

Does it look something like this?

http://ecx.images-amazon.com/images/I/31CTtSopLGL.jpg

If so, you could hacksaw 1/4" off each end and use a 1/2" spacer.

Yes thats exactly what it looks like. The other problem though, is that the axle does not fit into the fork either, the axle on the mount is about 3/8" and it needs to be around 5/16". However, this would probably make the mount swing around too much, and i need something that, when screwed in, isn't able to move at all. Are you sure there is no mount that is made for smaller bikes?

If the axle is too thick, you could file flats on it to fit the fork.

when you say I could file flats to fit the fork, do you mean cutting a small notch or groove in the axle at the location of the fork so the new size can accept the fork mount? if so, what tool can i use to do this?

A file.

IMO, it isn't worth the time and effort. I just bungee cord the bike in the truck with the tie downs.