Monitoring Tascam DR100 mkii

You are using a good set of cans to listen, right?

Most of the ops I know prefer it around their neck. When you're boom op and PSM it's tricky. Set your levels, then focus on the action and pointing the microphone. Listen for clipping or it being too low.

When I'm the lucky guy who gets to run camera AND audio (on top or lighting, directing and everything else on a 1 man crew shoot) I have a short arm that fits in my camera hot shoe and has a 1/4-20 screw on the other side. I mount the recorder on my camera at an angle where I can see the levels and buttons.

That's one of the big problems with those hand held recorders, they are designed to be hand held facing up. When I used to use one of those the best I could come up with was to try and rest it on a stool or table top and not pull it off. Of course only gonna work for nice static shots.
Sometimes I managed to kinda hold it along with my boom but that's not a great solution. Try and find a way of strapping that cute likkle case to the boom?
Apart from that is just allowing yourself shit loads of headroom.

Unless you have a specific need for the protection from the case I just drilled up some flat bar and screwed the unit to that with a srcew I nicked off an old tripod and attached a shoulder strap off a bag with a couple of key rings. Also put a rubber pad 'tween the recorder and flat bar for a bit of protection.