I can't tell from the pictures but does your compass/D3 have a slot for the strap to go into? My computer/compass both a hockey puck types that sit in boots with straps molded on.
I got a kit for the D3 so that you can use something other than the standard band with it. It was very inexpensive, like $10. I cut off the straps from the compass boot, and I just cut a slit through it with a knife at the base. I think that they make the boots for these with a slit already. I just couldn't find them.
I looked at the DiveRite Express website and couldn't find the elastic webbing with a buckle that you are referring too. Do you have a link to a source for this?