Herewith i want to share my design of a RC boat motor mount.
The motor mount is designed to mount a Graupner Bushless Speed 600 BB motor but can be used for numerous types of brushless and brushed motors with a mounting diameter of 25mm.
I've integrated slotted holes at the side walls can be used for fixing the motor wires with cable ties.
The motor angle can be adjusted 45 degrees.

Measured torque of the screws has to be arround 60Ncm, not more!

I've printed the parts with my Anycubic Photon DLP printer but the parts are designed that they can be printed with any other FDM printer.

Dimensions of the motor mount without holes for water cooling tubes:
Distance between the outer corners of the both side walls = 65mm
Distance between the inner corner of the both side walls = 55mm
Total length = 105mm
Total height with 90° mounted motor = 45mm

Dimensions of the motor mount with holes for water cooling tubes:
Distance between the outer corners of the both side walls = 75mm
Distance between the inner corner of the both side walls = 65mm
Total length = 105mm
Total height with 90° mounted motor = 45mm

Dimensions of the motor mount small:
Distance between the outer corners of the both side walls = 58mm
Distance between the inner corner of the both side walls = 50mm
Total length = 80mm
Total height with 90° mounted motor = 45mm

What you need:
4 x Hexagon head screw M3x16
4 x Hexagon nut M3
4 x Washer M3
4 x Hexagon head screw M3x8 to mount the motor