I made this thing secure a pair of my favorite relay boards on to a flat surface. Here is an example of the boards that will fit (they are well designed compared to most):
There are 2 different versions:
dualRelayMount.stl uses 8x M3 bolts to secure the boards.
dualRelayMount2.stl is hardware free, using melt-down pins to secure the boards. Heat a medium sized flathead screwdriver with a lighter and press down on the pins. If the pins turn 'gummy feeling' and want to come off with the screwdriver, you probably just need to get your screwdriver a little hotter. Version 2 is shown in the photo.
Both versions have 4x 4mm holes for mounting. No supports necessary for either as well.