Here's a 3" frame that was designed to hold a Insta360 GO camera underneath the main FPV camera for super-low over-the-ground shots, and also carry a LiPo battery on the top-plate.

In addition to that, it was designed with replaceable arms. This led to the decision of using M3 hardware since I find that M2 is not robust enough to withstand the stress of holding a frame together without stripping the standoffs.

To make this happen, I was inspired by a frame called the Babytooth drafted by Simba5 (thing:3843046), which uses screws, pressnuts, and TPU parts to sandwich the individual parts together.

For the overall design/layout of the arms, I was inspired by the RangeX that is sold by FPVFrame.ch

Hence the name BabyrangeX.

The build itself is not easy though, and there is basically only one option to use for VTX, which is the TBS Unify Pro32 Nano, or similar sized VTX. The RX also has to be similar size of a TBS Crossfire Nano RX.

A capacitor, the VTX, and RX find place in the front of the frame behind the camera, the FC (whoop board) goes in the rear. VTX antenna cable is routed below the FC to the rear.

I suggest using 1404 or 1207 motors for use with high pitched 3" props to be able to lift the weight of the entire quad.

Personally I use 3700KV 1404 motors with 3S batteries. This gives it a lot of efficiency and throttle-control.

The build is surprisingly easy to fly low over the ground. Already in the FPV cam, the video looks impressive, and on the Insta you get what I designed the frame for: low shots that scrape the ground.

It does not like to go fast with the motors I used though, especially with the weight of the Insta in the front.

Can be surprisingly fun to fly without the camera attached.

To build the frame, you will need:
Carbon:
1x topplate 1.5mm
1x bottomplate 2mm
2x sideplate 1mm
1x frontarms 2.5mm
1x sidearms 2.5mm

You can make the top/bottomplate thicker as you wish. However, if you want to use the sideplates, the thickness of the arms need to stay as designed, unless you go into the STP file and modify it yourself.

TPU:
1x rearassembly
1x frontassembly
1x frontassembly mirrored

Hardware:
4x M3 pressnuts
4x M3 16mm Screws

Since the TPU parts will be used to sandwich the frame, I suggest a high amount of infill, but more importantly, high amount of sidewalls.

Again: this is not an easy build. I would even say it's frustrating. The result however does what it's supposed to.