Looking forward to seeing the progress.
I'm no Rogue One guru but here are a few answers for you
1. The CRL says it mounts to the belt with no visible screws or clips. I'm assuming some of you rivet or screw it to the belt and do not use snaps. I've been reading a lot of build threads but have not come across that yet. Any help on good ways to attach the thermal detonator to the belt would be appreciated. Some mount with screws/bolts, some use magnets and some use velcro, so it can be removed and some even glue it on.
Which ever way you mount the detonator they can come off at times
2. I've come across several ways to fill seamss (e.g. bondo). I'll have to do that after I connect the two halves. I'm new to any kind of model work so I'll take whatever advice you have for a newbie's first attempt. Some use ABS paste, it just helps on places that may flex, it's a little more work though, JB weld is sandable and is also good for flexing pieces, bondo can crack under stress/bending.
3. The total length will be 8 1/4 inches. The CRL says approximately 8". I'm assuming my 1/4" extra is okay. I doubt your GML will want you to get a ruler out, should be fine